Author: Green Design and Build Team

An accessory dwelling unit is a second, complete home on a lot that already has a house on it. California law defines it as a unit with permanent provisions for living, sleeping, eating, cooking and sanitation — and that word “cooking” is the whole test. A structure with a bedroom and a bathroom but no kitchen is not an ADU. Add a kitchen and it is. Everything else people argue about, including whether to call it a granny flat, a casita or a mother-in-law suite, is vocabulary rather than law. Green Design and Build is a licensed general contractor (CSLB #1110975) based in Van Nuys that designs, permits and builds ADUs across Los Angeles County, Orange County, Ventura County, western Riverside County and western San Bernardino County.

We are led by Dekel Sofer, who has more than 16 years of hands-on construction experience; the company has completed hundreds of projects and holds a 4.7-star rating across 146 reviews on Yelp. The state rules below apply everywhere in California. The Los Angeles specifics are called out separately, because the local half of this varies by city.

The legal definition, and the one word that decides it

Under Government Code section 66313, an accessory dwelling unit is an attached or detached residential dwelling unit that provides complete independent living facilities for one or more people, on a lot with an existing or proposed primary residence. It must include permanent provisions for five things: living, sleeping, eating, cooking and sanitation. An efficiency unit and a manufactured home both count.

Four of those five are easy. The fifth is the line everything turns on. The City of Los Angeles makes it even blunter in its own zoning code, which defines a dwelling unit as a group of two or more rooms, one of which is a kitchen, designed for occupancy by one family. No kitchen, no dwelling unit. That single distinction decides whether your project is an ADU with state protection behind it, or an accessory building with none.

Every other name for an ADU

Most people searching for this do not use the letters A-D-U. They use a word they grew up with. None of the terms below is defined anywhere in California statute or in the Los Angeles Municipal Code; the state’s own housing department acknowledges as much, noting that ADUs “have been known by many names: granny flats, in-law units, backyard cottages, secondary units and more.”

What people call it	Legal status in California	What it usually means in practice
Granny flat	Colloquial	Any ADU, usually detached, often built for a parent
Mother-in-law suite / in-law suite	Colloquial	Usually an attached ADU or a junior ADU inside the house
Casita	Colloquial	A small detached unit, often Spanish or Mediterranean in style
Backyard cottage	Colloquial	A detached ADU
Secondary suite / secondary unit	Colloquial	An ADU. “Second unit” was California’s old statutory term
Carriage house	Colloquial	An ADU above or inside a detached garage
Guest house	Not an ADU	No kitchen. In Los Angeles this is a separate zoning category with its own rules — see below
Junior ADU (JADU)	Defined in statute	Up to 500 sq ft, inside the walls of the existing house
Movable tiny house	Defined by the City of Los Angeles	A DMV-registered unit on wheels, 150 to 430 sq ft

The practical consequence is simple: if you ask a building department for a granny flat, nobody will know what standard to apply to you. Ask for an accessory dwelling unit and a specific, generous body of state law attaches to your application.

The four kinds of ADU

Type	Where it goes	Worth knowing
Detached, new construction	A new structure in the yard	Cities must allow at least 800 sq ft with four-foot side and rear setbacks
Attached	Built onto the existing house	Shares a wall; height is tied to the main house
Conversion	Inside existing space — a garage, a basement, part of the house	No new setback required, and up to 150 sq ft of expansion is allowed purely for entry and exit
Junior ADU (JADU)	Within the walls of the single-family house, including an attached garage	500 sq ft maximum, and its own rulebook

Our guides compare a detached ADU against a garage conversion and weigh an ADU against a room addition if you are deciding between them.

What a junior ADU actually is

A JADU is no more than 500 square feet of interior livable space, contained entirely within the walls of a single-family residence. It is the cheapest way into an ADU because you are not building a structure, and it is the most misunderstood. The rules:

• One JADU per residential lot.
• It needs its own exterior entrance, separate from the main entrance to the house.
• It needs an efficiency kitchen: a cooking facility with appliances, plus a food preparation counter and storage cabinets sized reasonably for the unit. Notably, the state housing department has said cities may not dictate counter or cabinet sizes, specific electrical or gas connections, or appliance types.
• Sanitation can be shared with the house or separate.
• A deed restriction is recorded, prohibiting sale of the JADU separately from the house.
• Rentals must be longer than 30 days, so no short-term letting.
• Owner-occupancy changed on 1 January 2026. Under AB 1154, it is now required only where the JADU shares sanitation facilities with the house. Give the JADU its own bathroom and the owner-occupancy requirement falls away. Local ordinances written before 2026 may still show the old blanket rule; state law governs.

Our junior ADU cost guide covers what one costs to build.

How small can an ADU be?

Smaller than most people expect. California’s Health and Safety Code allows an efficiency unit with a minimum floor area of 150 square feet for occupancy by up to two people, and state ADU law separately forbids a city from setting a minimum ADU size that would prohibit an efficiency unit. So a 150-square-foot ADU is legal in principle anywhere in California.

You may see 190 square feet quoted as the minimum. There is no statute or code section behind that number and we can find no official source for it; it appears to be an error that has propagated across contractor websites. The figure that does exist alongside 150 is 220 square feet, which comes from the Building Code’s efficiency dwelling unit provisions, and the state has specifically rejected attempts to use it as an ADU minimum.

Separately, the Residential Code sets habitability floors that apply to any room: habitable rooms other than kitchens must be at least 70 square feet and at least 7 feet in any horizontal direction, with ceilings at least 7 feet high, or 6 feet 8 inches in a bathroom or laundry.

How many can you have on one lot?

On a single-family lot, three, as a floor rather than a ceiling. State law requires cities to ministerially approve a combination of: one ADU created within existing space, one junior ADU, and one newly constructed detached ADU. That is three additional units alongside the primary residence, and the state housing department’s own handbook states it in those terms. A city may allow more under its own ordinance; it may not allow fewer.

ADUs also do not count against your lot’s allowable density. Government Code section 66319 says an ADU is an accessory use that “shall not be considered to exceed the allowable density for the lot.” Do not over-read that, though: ADUs are still counted as housing units in the state’s regional housing reporting. It means your zoning does not block them, not that they are invisible.

A guest house is not an ADU, and in Los Angeles it has its own rules

This is where most published advice goes wrong, and it matters, because “can I just build a guest house instead” is one of the most common questions we get.

First, the Los Angeles Municipal Code does define “Guest House” — but not the way you think. In the LAMC a Guest House is a dwelling containing not more than five guest rooms or suites of rooms with no kitchen facilities. That is a lodging definition. The backyard building homeowners mean is a different defined term: Accessory Living Quarters, which the code describes as an accessory building used solely as the temporary dwelling of guests of the occupants, having no kitchen facilities and not rented or otherwise used as a separate dwelling unit.

You can still build one in Los Angeles. The catch is in the R1 zone rules: accessory living quarters may not be located on a lot smaller than 10,000 square feet. Most Valley and South Bay lots are well under that. And unlike an ADU, an accessory living quarters gets no state protection at all — no guaranteed four-foot setbacks, no ministerial approval, no minimum size the city must allow. You are entirely in the hands of local zoning.

What counts as a “kitchen” for this purpose is more expansive than a stove. The City’s zoning interpretation manual has historically treated a natural gas outlet, a 220-volt outlet, a double sink, a bar sink over one square foot, a hot water line to a bar sink, a refrigerator over 10 cubic feet, a garbage disposal, a dishwasher or the space for one, any device designed for cooking or heating food, and more than 10 square feet of counter as kitchen facilities. That is interpretive guidance rather than code text, and it dates from an older edition of the manual, so confirm the current position with the department — but it tells you how carefully this line is policed.

And if you build a kitchen-less guest room and quietly add a kitchen later? It stops meeting the accessory living quarters definition and becomes an unpermitted dwelling unit. That is a code enforcement problem, not an upgrade. The lawful route is to permit it as an ADU from the start — which, given the state protections an ADU carries and the 10,000-square-foot lot rule a guest house does not escape, is usually the easier path anyway.

Movable tiny houses

The City of Los Angeles recognises a category most cities do not. A Movable Tiny House is registered with the DMV, meets ANSI 119.5 or NFPA 1192 standards, cannot move under its own power, and measures between 150 and 430 square feet. In Los Angeles it can serve as an ADU. Treat this as a City of Los Angeles answer only: other jurisdictions handle tiny houses on wheels very differently and many do not accept them as ADUs at all.

What if there is already a unit back there?

Two different pathways, and they get blurred constantly.

A lawfully permitted structure with no kitchen — a guest room, a rec room, a garage — converts under the state’s conversion provision. The city must approve it ministerially, and you are allowed up to 150 square feet of expansion beyond the existing footprint purely to create entry and exit. The work is adding a compliant kitchen and bringing the building up to habitability standards.

An unpermitted unit someone has been living in is a different statute. Government Code section 66311.7 (which you may see cited by its old number, 66332) lets you legalise a unit built before 1 January 2020. The city cannot deny the permit for building-standard violations or for failing to meet the local ADU ordinance, and cannot charge impact fees. Los Angeles implements it through a documented process with a health-and-safety checklist, and the unit may comply either with current codes or with the codes in effect when it was built. You will need to evidence the construction date — utility bills, leases, contractor receipts.

The kitchen, in practice

This Los Angeles project is a garage conversion: the same footprint, a new kitchen with full appliances, a bathroom, and French doors where the garage door used to be. Once that kitchen went in, the building stopped being a garage and became a dwelling unit, with everything that follows from it. The full sequence is on the garage-to-ADU conversion project page.

How common are ADUs now?

Common enough to have changed what California builds. According to the state housing department’s March 2026 ADU handbook, ADUs permitted annually across California grew from 1,336 in 2016 to 30,354 in 2024, and in 2024 they made up more than 26.6 percent of all homes permitted statewide. Those are permits rather than finished buildings, so treat them as a measure of intent as much as output — but better than one in four homes permitted in California being an ADU is a genuine shift, and it is why the law keeps moving in their favour.

What it costs and how long it takes

Covered properly in their own guides rather than summarised badly here: what an ADU costs to build, what a garage conversion costs, how long an ADU takes, and how homeowners pay for them. If quotes you have received vary wildly, there are specific reasons for that.

For what your particular lot allows, we publish ADU rules by city across Southern California, an ADU rules calculator, and a guide to the LADBS standard plan program, which is the shortcut most homeowners miss.

Where we build ADUs

We build ADUs from our Van Nuys office across the San Fernando Valley, the rest of Los Angeles County, Orange County, Ventura County, western Riverside County and western San Bernardino County. The mix changes by area: garage conversions dominate in the older Valley neighbourhoods where detached garages sit at the back of the lot, detached new construction is more common on the wider lots of the Inland Empire and outer Orange County, and junior ADUs make the most sense in larger houses where a wing can be separated off. See our ADU construction service and all of our service areas.

ADU FAQs

What is an ADU?

An accessory dwelling unit is a second, self-contained home on a lot with an existing or proposed primary residence. California law requires it to have permanent provisions for living, sleeping, eating, cooking and sanitation. In practice the kitchen is the defining feature: a structure with a bedroom and bathroom but no kitchen is not an ADU. It can be detached, attached to the house, or created inside existing space such as a garage.

Is a granny flat the same as an ADU?

Yes. “Granny flat” is a colloquial term with no definition in California statute or in the Los Angeles Municipal Code. So are mother-in-law suite, in-law suite, casita, backyard cottage, secondary suite and carriage house. The state housing department itself notes ADUs have been known by many names. Use “accessory dwelling unit” when you apply for a permit, because that is the term the law attaches to.

What is the difference between an ADU and a guest house?

A kitchen. An ADU has one; a guest house does not, and without one it is not a dwelling unit. In Los Angeles the backyard version is formally called accessory living quarters, and it may not be built on a lot smaller than 10,000 square feet in the R1 zone. A guest house also gets none of the state protections an ADU receives, such as guaranteed four-foot setbacks and ministerial approval, so on most lots the ADU route is both easier and more useful.

What is a junior ADU?

A JADU is up to 500 square feet of interior livable space created entirely within the walls of a single-family house, including an attached garage. It needs its own exterior entrance and an efficiency kitchen with a cooking facility, appliances, a food preparation counter and storage. Sanitation may be shared or separate, and a deed restriction is recorded against the property. Since 1 January 2026, owner-occupancy is required only where the JADU shares a bathroom with the house.

How many ADUs can I build on my property?

On a single-family lot, state law requires cities to allow a combination of one ADU created within existing space, one junior ADU, and one newly built detached ADU — three additional units alongside the house. That is a minimum, not a maximum; a city may permit more. ADUs also do not count toward your lot’s allowable density.

How small can an ADU be?

California allows an efficiency unit with a minimum floor area of 150 square feet for up to two occupants, and a city may not set an ADU minimum size that would prohibit one. Habitability rules still apply: habitable rooms other than kitchens must be at least 70 square feet and 7 feet in any horizontal direction, with 7-foot ceilings. The 190-square-foot minimum widely quoted online has no statutory basis that we can find.

Can I rent out an ADU?

Yes, for terms longer than 30 days. Cities may require, and in practice nearly always do require, a minimum 30-day rental term, which rules out short-term letting. A JADU carries the same 30-day floor. An ADU generally cannot be sold separately from the main house, though a small number of California cities have opted into a condominium pathway that allows it.

Can I legalise an unpermitted unit in my backyard?

Often, yes, if it was built before 1 January 2020. Government Code section 66311.7 requires cities to permit qualifying unpermitted ADUs and junior ADUs without denying them for building-standard violations or for failing the local ADU ordinance, and without charging impact fees. The unit may comply with either current codes or the codes in effect when it was built, and a health-and-safety review applies. You will need documentation of the construction date.

Find out what your lot actually allows

We will look at the lot, the setbacks, the existing structures and your city’s current ordinance, and tell you which of the three routes — conversion, junior ADU or new detached unit — makes sense on your property before anyone draws anything. The site visit takes about two hours, the written proposal follows within 48 hours, and both are free. We reply within one business day.

A junior accessory dwelling unit (JADU) is a unit of no more than 500 square feet of interior livable space created entirely inside an existing or proposed single-family house, including an attached garage, with its own entrance and an efficiency kitchen; it may share a bathroom with the main house. Because a JADU reuses the roof, walls and floor you already have, it is usually the least expensive legal way to add a second unit, and it pays no development impact fees. Green Design and Build is a licensed, bonded and insured general contractor (CSLB #1110975) based in Van Nuys, building ADUs, JADUs and garage conversions for homeowners across Los Angeles County, Orange County, Ventura County, western Riverside County and western San Bernardino County (Ontario, Rancho Cucamonga and neighbouring cities).

We are led by Dekel Sofer; the company has completed hundreds of projects and holds a 4.7-star rating across 146 reviews on Yelp.

What a JADU is, in the words of the law

The rules for junior ADUs sit in California Government Code section 66333, and every city in our service area, including the City of Los Angeles, has to allow them. Verified against the Department of Housing and Community Development’s ADU Handbook (March 2026 edition), the definition has six parts:

1. Size. No more than 500 square feet. Since January 1, 2026 the statute measures “interior livable space,” so wall thickness does not count against you.
2. Location. Contained entirely within the single-family residence. An attached garage counts as part of the house, so converting an attached two-car garage into a JADU is the most common version we build. A detached garage cannot be a JADU; it becomes a garage conversion ADU instead.
3. Kitchen. An efficiency kitchen, which the statute defines as “a cooking facility with appliances, a food preparation counter, and storage cabinets that are of reasonable size in relation to the size of the JADU.” A city cannot demand a specific counter length, a 240-volt range circuit or a full-size stove.
4. Bathroom. A JADU “may include separate sanitation facilities or may share sanitation facilities with the existing structure.” If it shares, it must have its own exterior entrance and an interior door into the main living area.
5. Owner occupancy. Since January 1, 2026 (AB 1154), the owner must live in either the house or the JADU only when the JADU shares a bathroom with the house. Give the JADU its own bathroom and no owner-occupancy requirement applies. The requirement never applies when the owner is a government agency, land trust or housing organization.
6. Deed restriction. The city may require a recorded deed restriction that does two things and only two: prohibits selling the JADU separately from the house, and holds the unit to the size and attributes state law allows. The City of Los Angeles records it before the permit issues.

Two more rules matter. Only one JADU is allowed per lot. And no city may require parking for a JADU, even when it replaces an attached garage.

JADU plus ADU: the two-unit lot

The reason JADUs are worth knowing about is Government Code section 66323. On a single-family lot, a city must ministerially approve one JADU (or one ADU converted from existing space) together with one new detached ADU of up to 800 square feet with four-foot side and rear setbacks. That is how a standard 1950s Valley ranch house on a 6,000-square-foot lot ends up with three legal units: the house, a JADU in the attached garage, and a detached unit in the back yard. The detached unit is where most of the money and the rental value sit; the JADU is the inexpensive second one. Our ADU rules hub and the ADU rules calculator show the minimums your city has to allow for both.

How much does a JADU cost in Los Angeles?

A JADU in Los Angeles typically costs $60,000 to $150,000. That figure and every other on this page are typical Los Angeles budgeting estimates, not a price list or a quote. Your project can come in lower or higher depending on the house, the scope and the finishes; the only real number is a written proposal after a site visit.

The ceiling to compare against is the published range for a garage conversion ADU, $100,000 to $200,000, because a JADU built in an attached garage is the same shell with less plumbing: it can share the bathroom, and its kitchen is a cooking counter rather than a full kitchen with a range hood ducted through the roof. A JADU carved out of a bedroom wing with a new exterior door and no bathroom of its own sits at the low end; a garage JADU with its own bathroom, a new slab section for the drain, and a panel upgrade sits at the high end and overlaps the garage-conversion range.

Item	What it involves	What moves it
Structure and envelope	Insulating and sheathing garage walls, replacing the garage door with a framed wall and entry, new windows for egress and light	A garage slab with no vapor barrier has to be sealed or overlaid; sloped garage slabs need leveling
Efficiency kitchen	Counter, cabinets, sink, plug-in or small built-in appliances; a 20-amp circuit or two	A full range with a hood ducted outside turns it into a full kitchen and adds gas or 240-volt work
Bathroom	Optional; sharing the house bathroom removes the largest plumbing cost	Cutting the slab for a new drain, and where the sewer lateral runs
Electrical	New circuits, lighting to Title 24, smoke and carbon monoxide alarms	Many 1950s to 1970s houses need a 200-amp panel first; the published range for that upgrade is $6,000 to $8,000
Heating and cooling	A ductless mini-split heat pump is the usual answer	Panel capacity and the outdoor unit’s location
Fire separation	Rated drywall and a self-closing door between the JADU and the house where the code requires it	Garage-to-house walls usually already have the rated layer; ceilings often do not
Permits and deed restriction	LADBS or city plan check, the recorded covenant, inspections	No impact fees; connection fees are not charged for a unit inside existing space

The published ranges for other ADU types are in our guide to how much it costs to build an ADU on your property, and the garage figures are broken down in our garage conversion cost guide.

JADU vs ADU vs garage conversion

	Junior ADU	Garage conversion ADU	Detached ADU
Maximum size	500 sq ft interior livable space	Existing garage footprint (plus up to 150 sq ft for entry and egress)	800 sq ft guaranteed by state law; up to 1,200 sq ft in the City of Los Angeles and many other cities
Where	Inside the house or attached garage	Attached or detached garage	New building in the yard, 4 ft side and rear setbacks
Kitchen	Efficiency kitchen	Full kitchen	Full kitchen
Bathroom	Own or shared with the house	Own	Own
Owner must live on site	Only if the bathroom is shared	No	No
Deed restriction	Yes, in most cities	No	No
Impact fees	None	None (750 sq ft or less)	None at 750 sq ft or less; proportionate above
Replacement parking	None required	None required	None required
Fire sprinklers	Not required if the house has none	Not required if the house has none	Not required if the house has none
Solar	Not required	Not required	Required on new detached units in the City of Los Angeles
Can be combined with	One detached ADU up to 800 sq ft	One JADU	One JADU or one conversion ADU
Rental term	Longer than 30 days	Longer than 30 days	Longer than 30 days
Typical Los Angeles budgeting range	$60,000–$150,000	$100,000–$200,000	$180,000–$450,000+ by size

When a JADU beats a full ADU

A JADU is the right unit when the budget is the constraint, when the yard is not there, or when the second unit is for family rather than rent. It makes sense for a parent or an adult child who will share the house’s bathroom without friction, for a lot where a detached unit would eat the only outdoor space, and for a homeowner who wants a second permitted unit in months rather than a year. It is also the right first step on a lot that will eventually get a detached ADU too, because the JADU approval does not use up the detached-unit entitlement.

A full ADU is the better answer when rental income is the goal, because a 500-square-foot unit that shares a bathroom and has a hot plate rents for far less than a self-contained unit with its own kitchen and bath; when the only convertible space is a detached garage, which can never be a JADU; and when you do not want a deed restriction or an owner-occupancy condition on title. Our guides to detached ADU vs garage conversion and ADU vs room addition cover the other two decisions on the same lot.

Permits and the 60-day clock

A JADU is a ministerial permit, the same as an ADU. In the City of Los Angeles, including Van Nuys, Sherman Oaks, Encino, Studio City, Woodland Hills, Northridge and North Hollywood, the plans go to LADBS; Burbank, Glendale, Pasadena, Santa Clarita, every Orange County city and the western Riverside and San Bernardino County cities each run their own building divisions under the same state law. The agency has 15 business days to tell you whether the application is complete and then 60 days to approve or deny it. The deed restriction is recorded with the county before the permit issues, and the inspections are the ordinary building, electrical, plumbing and mechanical inspections plus the final.

Inside the house the code issues are practical rather than legal: a bedroom used as a JADU still needs an egress window, the wall and ceiling between an attached garage JADU and the house need the rated drywall and the self-closing door the residential code requires for a garage, and every sleeping room gets interconnected smoke alarms. None of it is hard; all of it should be on the drawings before plan check rather than discovered by the inspector.

How long does a JADU take?

A JADU is the fastest legal second unit because there is no foundation, no roof and usually no utility trenching. Design and drawings take a few weeks, plan check runs inside the statutory 60 days when the submittal is complete, and construction on an attached-garage JADU is measured in weeks rather than months: framing the door opening, rough electrical and any plumbing, insulation, drywall, the kitchen counter, flooring and finish. Six months to a year is the published range for an ADU project overall; a JADU sits at the short end of it, and our how long does an ADU take guide breaks the phases down.

A real garage that became a unit

Our garage-to-ADU conversion in Los Angeles, finished in 2025, shows what the fuller version of this work looks like: a detached two-car garage of about 500 square feet by the pool, with the roof structure rebuilt, new framing and sheathing, new windows and French doors where the garage door had been, stucco to match the tile roof, a full kitchen with an island, a tiled bathroom with a glass shower and wood-look flooring throughout, now a long-term rental. Because the garage was detached it had to be a full ADU rather than a JADU, with its own bathroom and a complete kitchen. A JADU in an attached garage keeps most of that scope and drops the parts that cost the most: the second bathroom’s plumbing and the full kitchen.

The other lesson comes from why ADU quotes vary so much between contractors: a garage conversion estimated at $85,000 to $95,000 that finished near $130,000 after the slab turned out to have no moisture barrier, the 100-amp panel had to become 200 amps, and a camera found root damage in the sewer line. Every one of those applies to an attached-garage JADU too, which is why our site walk starts with the slab, the panel and the sewer.

Where we build JADUs

We build JADUs and ADUs from our Van Nuys office across the San Fernando Valley and the rest of Los Angeles County, where the attached-garage ranch house is the natural JADU candidate; in Orange County, where master-planned communities add HOA review that state law limits but does not remove; in Ventura County; and in western Riverside County and western San Bernardino County, where newer slab-on-grade houses with three-car garages leave room for both a JADU and parking. See every city we serve on our service areas page, and the full picture of what we build on our ADU page and garage conversion page.

Junior ADU FAQs

How much does a junior ADU cost in Los Angeles?

A junior ADU in Los Angeles typically costs $60,000 to $150,000, a typical Los Angeles budgeting range and not a quote; a garage conversion ADU, the closest comparison, runs $100,000 to $200,000 in our published ranges. A JADU that shares the house bathroom and uses a plug-in efficiency kitchen sits at the low end; one with its own bathroom, a slab cut for a new drain and a panel upgrade sits at the high end. These are estimates, not a price list, and the only real number is a written proposal after a site visit.

What is the difference between a JADU and an ADU?

A JADU is limited to 500 square feet of interior livable space, must be inside the existing house or attached garage, needs only an efficiency kitchen, may share a bathroom with the house, and carries a deed restriction and, if the bathroom is shared, an owner-occupancy requirement. An ADU is a fully independent unit with its own kitchen and bathroom, can be detached or converted from a detached garage, can be larger, and has no owner-occupancy condition.

Do I have to live on the property if I build a JADU?

Only if the JADU shares a bathroom with the main house. Since January 1, 2026, California Government Code section 66333 lets a city require owner occupancy of a junior ADU only when the unit shares sanitation facilities with the house; a JADU with its own bathroom carries no owner-occupancy requirement. Some cities still record a covenant on every JADU, so ask before you file.

Can I have a JADU and a detached ADU on the same lot?

Yes. Government Code section 66323 requires cities to approve, on a single-family lot, one junior ADU or one ADU converted from existing space together with one new detached ADU of up to 800 square feet with four-foot side and rear setbacks. A JADU in the attached garage and a detached unit in the back yard is the most common two-unit layout we build in the San Fernando Valley.

Does a JADU need its own kitchen and bathroom?

A JADU needs an efficiency kitchen, which state law defines as a cooking facility with appliances, a food preparation counter and storage cabinets of a reasonable size for the unit; it does not need a full range or hood. It does not need its own bathroom and may share the house’s, in which case it must have a separate exterior entrance and an interior door to the main living area.

Can I convert my detached garage into a JADU?

No. A junior ADU must be contained entirely within the single-family residence, and an attached garage counts as part of the house but a detached garage does not. A detached garage becomes a garage conversion ADU instead, with a full kitchen and its own bathroom, which is a larger scope with a published Los Angeles range of $100,000 to $200,000.

Find out whether your garage should be a JADU or an ADU

Send us your address and a photo of the garage or the room you have in mind. We will walk the lot, check the slab, the panel and the sewer, tell you which unit the law and the house allow, and give you a written, itemized estimate. The site visit and the estimate are free.

A garage conversion ADU turns the slab, walls and roof you already have into a legal dwelling and typically costs $100,000 to $200,000 in Los Angeles; a detached ADU is a new building in the yard, costs $180,000 to $260,000 for about 500 square feet and $300,000 to $450,000 or more for about 1,000 square feet, and gives you a unit whose size, ceiling height, layout and position you choose. Green Design and Build is a licensed general contractor (CSLB #1110975) based in Van Nuys that builds both for homeowners across Los Angeles County, Orange County, Ventura County, western Riverside County and western San Bernardino County (Ontario, Rancho Cucamonga and neighbouring cities). The conversion wins on cost and speed when the garage is sound and where you want the unit; the detached ADU wins when the garage is the wrong size, height or place, or when you want to keep the parking. These are typical Los Angeles budgeting estimates, not a price list or a quote, and your project can come in lower or higher.

We are led by Dekel Sofer. We hold a 4.7-star rating across 146 Yelp reviews and have completed hundreds of projects across all five counties. Our garage conversion and ADU pages describe each service; this guide is the comparison.

Garage conversion vs. detached ADU at a glance

	Garage conversion ADU	Detached ADU
What you get	The existing garage footprint (about 400 sq ft for a two-car garage), plus up to 150 sq ft of expansion for entry and egress under state law	A new unit of the size you choose: state law guarantees 800 sq ft, and the City of Los Angeles allows up to 1,200 sq ft where the lot’s standards permit
Typical Los Angeles budgeting estimate	$100,000–$200,000	About 500 sq ft $180,000–$260,000; 700–800 sq ft $225,000–$350,000; about 1,000 sq ft $300,000–$450,000+; $300–$450 per sq ft all-in
Setbacks	None required for a conversion within the existing footprint (or a rebuild to the same dimensions in the same location)	4 ft from side and rear property lines
Height	The garage’s existing height; ceilings must be at least 7 ft	At least 16 ft, or 18 ft within half a mile walking distance of a major transit stop; two stories where the city allows
Parking	The city cannot require the lost garage parking to be replaced	No parking can be required within half a mile of transit; otherwise at most one space
Fire sprinklers	Not required if the main house has none	Same rule
Solar	Not required for a conversion	New detached ADUs must have solar panels under the state energy code
LADBS standard plans	Not applicable; conversions go through regular plan check with their own drawings	Available for detached units in the City of Los Angeles, which limits plan check to site-specific review
Utility connection	A city or utility cannot require a new or separate connection for an ADU within existing space; shares the house’s water, sewer and panel	May be charged a connection fee proportionate to the unit; usually still fed from the house’s panel after a 200-amp upgrade
Timeline	Shorter end of six months to a year; construction about three to five months	Longer end of six months to a year
What you lose	Covered parking and storage	Yard space; the garage stays
What you cannot change	Slab elevation, ceiling height, footprint position and the original footings	Nothing; it is designed from scratch

These are typical Los Angeles budgeting estimates, not a price list or a quote. Your project can come in lower or higher depending on the house, the scope and the finishes; the only real number is a written proposal after a site visit. The figures are the ones on our homepage and on our garage conversion cost guide and ADU cost guide.

Cost: the garage is a head start, not a discount

A two-car garage conversion at $100,000 to $200,000 against a 500-square-foot detached ADU at $180,000 to $260,000 looks like a clear win for the conversion, and on a sound garage it is. The savings are specific: no foundation, no framing, no roof, no solar array. What you still pay for is everything that makes a garage a home, and a 400-square-foot ADU contains the two most expensive rooms in any house, a kitchen and a full bathroom. A $100,000 to $150,000 conversion of a 400-square-foot garage works out to roughly $250 to $375 per square foot, below the $300 to $450 we publish for new construction; but the ranges overlap, because a detached garage at the back of a deep Valley lot with a cracked slab, a 100-amp panel and a clay sewer lateral can cost as much as a small new unit. What moves either number is the same short list: slab or foundation, the sewer run and the lateral’s condition, the 200-amp service upgrade most 1950s to 1970s houses need, the envelope, and the kitchen and bathroom, which cost the same on both routes. Our garage conversion cost guide itemizes each.

Two real projects, one from each side

The conversion that rose 40 percent. In why ADU quotes vary so much between contractors, a homeowner converting a detached two-car garage of roughly 400 square feet into a one-bedroom ADU with a kitchen, bathroom and laundry started with an estimate of $85,000 to $95,000. Three discoveries moved it to about $130,000: the slab had no proper moisture barrier and needed preparation and concrete work; the home’s original 100-amp panel had to become 200 amps; and a camera inspection found root intrusion and bellying in the sewer, so a section had to be replaced before the new fixtures could connect. None of that was a luxury; each was a condition the garage already had.

The detached unit that went from $180,000 to $265,000. In our ADU cost guide, homeowners who had already removed an old, deteriorated garage planned a roughly 600-square-foot one-bedroom detached ADU on a budget of about $180,000; it finished near $265,000 after a $14,000 clay sewer reroute under the new foundation, a $6,000 to $8,000 panel upgrade with about three weeks of utility delay, and about $30,000 of finish upgrades chosen after framing. It is also the third option this guide keeps coming back to: when the garage is too far gone to convert, demolish it and build new on the same spot.

A finished conversion. Our garage-to-ADU conversion in Los Angeles, completed in 2025, started as a plain stucco box of about 500 square feet with one wide door opening and a single small window and became a self-contained rental with a rebuilt roof structure, new framing and sheathing, new windows and French doors, matched stucco, a full kitchen with an island and a full bathroom with a tiled shower. Notice that the roof structure was rebuilt, which is common once an old garage roof is asked to carry insulation, drywall and a ceiling.

The rules: where each one is allowed

California’s ADU statute lives in Government Code sections 66310 to 66342, and the rules below are verified against it and against LADBS; sources are at the end.

Conversions: no setback, no replacement parking, no separate connection

A garage converted to an ADU within its existing footprint needs no setback; section 66314(d)(7) says none shall be required for “an existing living area or accessory structure or a structure constructed in the same location and to the same dimensions as an existing structure” that is converted, and section 66323(a)(1) lets the unit expand up to 150 square feet for entry and exit. A garage one foot off the rear property line can become an ADU where a new building could not, provided the wall assembly handles fire separation. When a garage, carport or covered parking is converted or demolished for an ADU, section 66314(d)(11) says the city “shall not require that those offstreet parking spaces be replaced”; LADBS puts it plainly: “If you remove covered parking to build an ADU, you don’t need to replace it.” For an ADU within existing space, section 66324 bars the city or the utility from requiring a new or separate utility connection unless the unit is built with a new house. Sprinklers follow the main house on both routes.

Detached: 800 square feet guaranteed, four-foot setbacks, height, solar

For a new detached unit, section 66323(a)(2) requires ministerial approval of one detached ADU of up to 800 square feet with four-foot side and rear setbacks, and section 66321 bars a city from applying any floor-area, lot-coverage or open-space standard that would prevent it. Height is at least 16 feet, or 18 feet within half a mile walking distance of a major transit stop, and under the City of Los Angeles ordinance a detached ADU can go to 1,200 square feet where the lot’s own standards allow; our Los Angeles ADU rules page and ADU rules calculator show what applies to your address. LADBS states that “detached ADUs built from scratch must have solar panels,” a state energy-code requirement a conversion does not carry. An ADU of 750 square feet or less is exempt from local impact fees on either route.

Standard plans and the 60-day clock

The LADBS ADU Standard Plan Program pre-approves complete designs for detached units, roughly 60 plans from 200 to 1,200 square feet including the city-owned YOU-ADU plan that owners may use free of charge, and plan check is then limited to site-specific items such as zoning and the foundation. A conversion starts from the garage you already have, so it goes through regular plan check with its own drawings; our guide to ADU standard plans in Los Angeles explains when the catalogue helps. Both routes get the same deadline under section 66317: the city must confirm within 15 business days that an application is complete and approve or deny it within 60 days, or it is deemed approved, with no hearing or neighbour notice.

What a garage conversion cannot fix

This is what decides most projects, and it is why the site walk comes before the price. Los Angeles County Building and Safety publishes standard notes for converting an existing single-story garage, and the same state building code applies inside the City of Los Angeles and in every other city we work in. Some of what those notes require can be fixed with money; some cannot be fixed at all.

• Ceiling height. Habitable rooms need a ceiling of at least 7 feet (California Residential Code R305.1). Most garages clear it at the plate but not always once a finished ceiling, insulation and a level floor go in. Raising the roof is possible, as on the 2025 project above, but at that point you are paying for framing and a roof anyway.
• Slab elevation and condition. The garage slab was poured to hold a car: it slopes toward the door, often sits below the house’s finished floor, and on older garages has no vapor barrier. The County notes allow a sound 3-inch slab as is and a sound 2-inch slab only with a 2-inch concrete topping, with an approved moisture barrier over it; a cracked, heaved or thin slab gets replaced. What cannot change is where the slab sits relative to the yard and the house, which fixes the floor level, the drainage and the step at the door.
• Footings. A garage’s continuous footing was sized for a garage. The County requires the pre-construction meeting to include “exposing a section of the foundation to verify existing footing depth” for the added anchoring and bracing, and the garage-door opening “shall be converted to a permanent wall and a new foundation at this location shall be installed and anchored to the existing continuous footing.” If the footing turns out to be shallow or broken, the conversion becomes a partial rebuild.
• Footprint and position. The unit sits where the garage sits, at the garage’s size plus 150 square feet. If the garage is 300 square feet or backs onto the neighbour’s bedroom, no design fixes that.
• Envelope and openings. Walls, roof and floor have to reach the residential energy standard (R-21 in 2×6 or R-15 in 2×4 walls in the County notes), a bedroom needs an egress window of 5.7 square feet net clear opening, and the unit needs light and ventilation openings a garage never had. On an attached garage the wall to the house becomes a one-hour fire-rated assembly from slab to roof sheathing.
• Sewer. The drain has to reach the lateral with fall; the County notes call for the connection at least 24 inches outside the existing foundation at a 2 percent slope. A garage downhill from the sewer, or far from it, means a pump or a long trench.
• Junior ADU (JADU) rules and cost in Los Angeles

None of this means conversions are a bad idea. It means the garage has to be looked at before it is priced, and the honest outcome of a site walk is sometimes “demolish and build new on the same footprint,” which keeps the no-setback benefit while solving the height, slab and footing problems at once.

Rent and value: what each one is worth

Tenants pay for a kitchen, a bathroom, light and privacy, not for how the building started, so a properly done conversion tends to rent like a detached unit of the same size, and a compromised one (low ceiling, small windows, a step down at the door) tends to rent for less. A larger detached unit with a second bedroom rents for more than any two-car garage can, and it can be placed for privacy from the main house in a way a garage on the driveway cannot. We do not publish rent figures because they vary by neighbourhood; HUD’s Fair Market Rents for Los Angeles County are a conservative benchmark, and our ADU financing guide shows how to compare net rent with the loan payment. On property tax, both routes are treated the same way: the county assessor values the new construction as of completion and adds only that amount, and the existing house is not reassessed. At resale, either unit sells with the house as a property with an income unit; a conversion also removes covered parking, which some buyers care about more than others.

When each one wins

Convert the garage when it is structurally sound, with a good slab, adequate ceiling height and a roof worth keeping; its size and position already suit a studio or one-bedroom of about 400 square feet; the lot has no room for a new building four feet off the lines, or the yard matters more to you than the garage; budget is the constraint and $100,000 to $200,000 is the number you can finance; or you want the shorter schedule.

Build a detached ADU when you want two bedrooms, higher ceilings or a unit over 550 square feet; the garage is too low, too small, badly placed or in poor condition, so a conversion would need a new roof, slab and footings anyway; you want to keep the garage for parking and storage; the lot is deep enough for a private unit away from the house; or rental value is the goal and a larger unit will carry the higher cost.

Demolish and rebuild on the garage footprint when the garage is in the right place but the wrong condition; a structure “constructed in the same location and to the same dimensions” keeps the no-setback benefit.

Run your address through the ADU rules calculator before you decide, and expect the site walk to check the slab, the ceiling, the footings, the panel and the sewer before any number is written.

Where we do this work

The choice between converting and building new shifts with the housing stock. Across the San Fernando Valley and the City of Los Angeles, 1950s to 1970s ranch houses come with detached two-car garages at the back of the lot that convert well when the slab and roof are sound, LADBS reviews both routes, and the standard plan catalogue is available for detached units. In Orange County, attached garages on 1970s to 1990s tract houses are common, HOA review applies to either project, and keeping covered parking often tips the decision toward a detached unit. Ventura County cities such as Thousand Oaks and Simi Valley run their own building divisions, and hillside lots often need a soils report for a new foundation. In western Riverside County and western San Bernardino County, the larger lots of Corona, Ontario and Rancho Cucamonga make both routes feasible, and expansive soils can call for a geotechnical report on a detached unit. See all of our service areas.

Detached ADU vs. garage conversion FAQs

Is it cheaper to convert a garage or build a detached ADU?

Converting a garage is usually cheaper: a garage conversion ADU in Los Angeles typically runs $100,000 to $200,000, while a detached ADU runs $180,000 to $260,000 for about 500 square feet and $300,000 to $450,000 or more for about 1,000 square feet. The gap narrows when the garage needs a new slab, footings or roof. These are typical Los Angeles budgeting estimates, not a price list or a quote; a real project can come in lower or higher.

Do I have to replace the parking if I convert my garage into an ADU?

No. Under California Government Code section 66314(d)(11), when a garage, carport or covered parking structure is converted to an ADU or demolished to build one, the city cannot require the lost off-street parking spaces to be replaced, and LADBS states the same rule for the City of Los Angeles. A detached ADU that leaves the garage standing keeps your covered parking.

Does a garage conversion ADU need setbacks?

No. A garage converted to an ADU within its existing footprint, or rebuilt to the same dimensions in the same location, needs no setback under Government Code section 66314(d)(7), and it may add up to 150 square feet for entry and exit under section 66323. A new detached ADU must sit at least four feet from the side and rear property lines. This is why a garage on the property line can become an ADU where a new building could not.

What can’t a garage conversion fix?

A garage conversion cannot change the slab’s elevation, the footprint’s size beyond 150 square feet of expansion, or its position on the lot, and it inherits the original footings and ceiling height. Habitable rooms need a ceiling of at least 7 feet, the garage-door opening must get a new foundation tied to the existing footing, and a thin or cracked slab needs topping or replacement. When those add up, demolishing and rebuilding on the same footprint keeps the no-setback benefit and solves all of them.

Does a detached ADU need solar panels?

Yes. LADBS states that detached ADUs built from scratch must have solar panels under the California energy code, while a garage conversion ADU does not carry that requirement because it is a conversion of an existing structure. The solar system is one of the items that makes a detached unit cost more than a conversion, alongside the new foundation, framing and roof.

Can I use an LADBS standard plan for a garage conversion?

No. The LADBS ADU Standard Plan Program pre-approves complete designs for new detached ADUs, so plan check is limited to site-specific review of zoning and foundation; a garage conversion starts from the garage you already have and goes through regular plan check with its own drawings. Both routes are covered by the state’s 60-day approval deadline for a complete application.

Convert it, rebuild it, or build new? Let us look at the garage first.

We will measure the ceiling, check the slab and expose the footing if needed, camera the sewer, check the panel and the setbacks, and tell you in writing whether your garage is worth converting, with a budgeting range for each route. The site visit and the estimate are free.

A room addition makes the house you live in bigger; an accessory dwelling unit (ADU) is a separate, self-contained home with its own kitchen, bathroom and entrance that can be rented, house a relative, or sit empty until you need it. In Los Angeles the two cost about the same per square foot, but they are governed by different rules, take different amounts of time, and answer different questions: an addition is the right choice when the new space must connect to your existing rooms, and an ADU is the right choice when you want income, independence for the occupant, or square footage your lot’s zoning would not otherwise allow. Green Design and Build is a licensed general contractor (CSLB #1110975) based in Van Nuys that designs and builds both, for homeowners across Los Angeles County, Orange County, Ventura County, western Riverside County and western San Bernardino County (Ontario, Rancho Cucamonga and neighbouring cities).

We are led by Dekel Sofer. We hold a 4.7-star rating across 146 Yelp reviews and have completed hundreds of projects across all five counties. Because we build room additions and ADUs with the same team, we have no reason to steer you toward one; the first job on a site walk is telling you which one your lot and your goal actually call for.

ADU vs. room addition at a glance

	Room addition	ADU
What it is	New square footage attached to and opening into the existing house	A separate dwelling with its own kitchen, bath and entrance: detached, attached, garage conversion or junior ADU inside the house
Best use	Primary suite, family room, kitchen expansion, second story; space you use every day	Rental income, a parent or adult child, a guest suite, a home office with privacy, future flexibility
Typical Los Angeles budgeting estimate	Ground floor $300–$500 per sq ft; second story $400–$650 per sq ft; $75,000 for a small bedroom to $650,000+ for a full second story	Garage conversion $100,000–$200,000; detached about 500 sq ft $180,000–$260,000; 700–800 sq ft $225,000–$350,000; about 1,000 sq ft $300,000–$450,000+; new construction $300–$450 per sq ft all-in
Zoning in the City of Los Angeles (R1)	Must fit the lot’s setbacks (typically 20 ft or prevailing front, 5 ft sides, 15 ft rear) and the 45 percent floor-area cap of the Baseline Mansionization Ordinance	State law requires approval of a detached ADU of up to 800 sq ft with 4 ft side and rear setbacks and 16 ft height (18 ft near major transit) regardless of the lot’s floor-area cap; the city allows up to 1,200 sq ft detached
Permit path	Building permit with full plan check, plus plumbing, electrical and mechanical permits; discretionary review in an HPOZ or hillside area	Ministerial approval: the city must approve or deny a complete application within 60 days; no hearing or neighbour notice
Timeline, start to finish	9–14 months	6–12 months
Can it be rented separately?	No; it is part of the house	Yes, on leases longer than 30 days; owner occupancy is not required for a standard ADU
Property tax	New construction is assessed and added; the existing house is not reassessed	Same rule: the ADU is assessed as new construction and added; the existing house keeps its base
Utilities	Extends the house’s systems; may need a panel upgrade	Usually shares the house’s water, sewer and panel; a 200-amp upgrade is common; separate electric meter optional
Resale	Sells as a larger house; value follows comparable larger homes on the street	Sells with the house as a property with an income unit; cannot be sold separately unless your city adopts a condominium ordinance under state law

These are typical Los Angeles budgeting estimates, not a price list or a quote. Your project can come in lower or higher depending on the house, the scope and the finishes; the only real number is a written proposal after a site visit. The addition figures are from our room addition cost guide and the ADU figures from our ADU cost guide.

Cost: close per square foot, different in total

Per square foot the two are close: $300 to $500 all-in for a ground-floor addition, $300 to $450 for a detached ADU, and past $500 for either on a difficult lot. The total is where they separate, because they are rarely the same size.

Project	Typical size	Typical Los Angeles budgeting estimate
Bedroom or home office addition, ground floor	200–350 sq ft	$75,000–$150,000
Primary suite with bathroom, ground floor	350–500 sq ft	$130,000–$250,000
Family room or kitchen expansion	300–500 sq ft	$110,000–$230,000
Partial second story	400–600 sq ft	$200,000–$350,000
Full second story	800–1,200 sq ft	$350,000–$650,000+
Garage conversion ADU	About 400 sq ft (two-car garage)	$100,000–$200,000
Detached ADU	About 500 sq ft	$180,000–$260,000
Detached ADU	700–800 sq ft	$225,000–$350,000
Detached ADU	About 1,000 sq ft	$300,000–$450,000+

These are typical Los Angeles budgeting estimates, not a price list or a quote. Your project can come in lower or higher depending on the house, the scope and the finishes; the only real number is a written proposal after a site visit.

An ADU always contains the two most expensive rooms in any house, a kitchen and a full bathroom, so a 400-square-foot ADU carries more plumbing, electrical and cabinetry than a 400-square-foot family room. On the other hand, a garage conversion starts with a slab, walls and a roof, which is why it sits at the bottom of every ADU range, and a detached unit does not disturb the house you live in, so there is no cost for tying a new roof into an old one or opening a bearing wall. A second-story addition is the most expensive space per square foot because the roof comes off and the framing below often has to be strengthened to carry it.

On either project, hold a contingency of 15 to 20 percent above the accepted bid, especially on a pre-1980 house. The 600-square-foot detached ADU retold in our ADU cost guide went from a $180,000 budget to about $265,000 after a clay sewer line under the new foundation had to be rerouted (about $14,000), the 100-amp panel became 200 amps ($6,000 to $8,000 and about three weeks waiting on the utility), and the owners added roughly $30,000 of finish upgrades. Every one of those can happen on an addition.

Rules: the ADU can go where the addition cannot

This is the difference that decides more projects than cost does.

The addition has to fit the zoning

In the City of Los Angeles, which includes most of the San Fernando Valley, a room addition on a typical R1 lot has to sit inside the lot’s setbacks, usually 20 feet or the prevailing setback at the front, 5 feet from each side property line and 15 feet from the rear, and under the Baseline Mansionization Ordinance’s cap on total floor area, 45 percent of the lot area on most R1 lots. A 1,500-square-foot house on a 6,000-square-foot lot is at 25 percent and has room; a 2,700-square-foot house on the same lot is at the limit and cannot add a room, regardless of what the yard looks like. Every addition goes through full plan check with plumbing, electrical and mechanical permits; an addition in a Historic Preservation Overlay Zone goes through the Office of Historic Resources, and hillside lots add grading review and usually a geotechnical report. Our room additions page walks through these rules.

The ADU is protected by state law

California Government Code section 66323 requires the city to approve, ministerially, one detached new-construction ADU of up to 800 square feet with four-foot side and rear setbacks on a lot with a single-family home, and section 66321 bars a city from applying any development standard, including floor-area ratio and lot coverage, that would prevent it. Height is at least 16 feet for a detached unit, 18 feet within half a mile walking distance of a major transit stop, and up to 25 feet or the zone’s limit for an attached unit; under the City of Los Angeles ordinance a detached ADU can go to 1,200 square feet where the lot’s own standards allow. A garage converted to an ADU needs no setback at all, and the city cannot require replacement parking. On a lot at its 45 percent limit, the ADU is the only new square footage available. Run your address through our ADU rules calculator or read the Los Angeles ADU rules page.

The permit clock

Section 66317 requires the city to tell you within 15 business days whether an ADU application is complete and to approve or deny a complete application within 60 days; if it does neither, “the application shall be deemed approved.” There is no equivalent deadline for a room addition, which is why plan check for an addition runs two to four months in our published timeline and a complete ADU submittal is usually through faster. Both wait on the same utility clearances.

Attached ADU versus addition: the grey area

An attached ADU is physically an addition with its own entrance, kitchen and bath, and a wall between it and the house. State law caps it at 50 percent of the existing house’s floor area, with an 800-square-foot floor the city must allow, and lets it rise to 25 feet or the zone height limit, whichever is lower. Homeowners who want a suite for a parent today and a rental in ten years often build an attached ADU rather than a primary-suite addition; the kitchen and the separate entrance are what make the space rentable later, and the cost of the kitchen is the price of that option.

Timeline: 9–14 months versus 6–12 months

Our published schedule for a room addition is 9 to 14 months from first meeting to final inspection: 6 to 10 weeks of design, engineering and Title 24 for a ground-floor addition (8 to 12 for a second story), 2 to 4 months of plan check, and 4 to 7 months of construction (5 to 7 for a second story). An ADU runs 6 months to a year, garage conversions at the short end and large detached units at the long end; our guide to how long an ADU takes breaks it down phase by phase.

The gap comes from two places. Plan check is shorter for an ADU because of the 60-day rule, and in the City of Los Angeles a detached unit can use one of the LADBS pre-approved standard plans, which limits plan check to site-specific items. Construction is also usually shorter for a detached ADU than for an addition of the same size because the crew is not working inside an occupied house: no protecting the existing rooms, no tying into a kitchen in use, no roof open over the family. A second-story addition is the extreme case, where the house is often unlivable for part of the build.

Rent, family and resale: what each one is for

An addition is used by the people who own the house, and its value at resale is the value of a larger house on that street; a primary suite or family room in a 1,200-square-foot Valley ranch house tends to bring it in line with the larger homes nearby, but it produces no income. An ADU is a separate dwelling: it can be rented long term (state law bars a city from requiring owner occupancy for a standard ADU, and rentals of state-exemption ADUs and junior ADUs must be longer than 30 days), it can house a parent with a real front door between the generations, and it can revert to family use later. At resale it is marketed as a property with an income unit, but it stays part of the same parcel; state law only lets an ADU be sold separately as a condominium where the city has adopted an ordinance under Government Code section 66342.

The property-tax treatment is the same for both: new construction is assessed at market value when completed and added as a supplemental assessment, and the existing house and land are not reassessed, so your Proposition 13 base stays where it is. Rent is the factor we will not put a number on; HUD’s Fair Market Rents for Los Angeles County and a local property manager are the right sources, and our ADU financing guide shows how to compare net rent against the loan payment.

How to decide

Work through these in order; most homeowners have an answer by the third question.

1. Does the new space have to open into the rooms you already have? A bigger kitchen, a family room off the living room, a second bathroom for the kids’ bedrooms: addition. Space used by someone else, or by you with a door between it and the house: ADU.
2. Do you want income, now or later? If yes, ADU; an addition never pays rent. If the answer is “maybe in ten years,” an attached ADU or a detached unit with a real kitchen keeps the option.
3. Is the lot at its floor-area limit or short on setbacks? If an addition does not fit under the 45 percent cap and the setbacks, the 800-square-foot ADU exemption is the square footage the state guarantees you.
4. Is there a garage you do not need for cars? A two-car garage of about 400 square feet is the lowest-cost route to an ADU, and the city cannot make you replace the parking. If you want the garage as part of the house instead, that is an addition and follows the addition rules.
5. Who is going to live in it? A parent who needs a bedroom near the family: addition or junior ADU inside the house. A parent who wants independence, an adult child, or a tenant: separate unit.
6. Can you live in the house during construction? An ADU does not disturb the house; a second-story addition is the slowest and most disruptive option and often means moving out for part of the build.
7. What will the neighbourhood pay for? On a street of 2,500-square-foot houses, taking a 1,300-square-foot house to 2,000 is value the market recognizes. On a street of small houses on large lots near transit, an income unit is what buyers pay for.

Can you do both?

Yes, and on many lots it is the right answer. State law allows a detached ADU of up to 800 square feet regardless of the lot’s floor-area cap, and a junior ADU of up to 500 square feet inside the house alongside it, so a homeowner can add a primary suite under the zoning rules and still build a detached unit in the yard under the ADU rules. One design and engineering package that covers both saves a second round of plan check, and the 200-amp panel and any sewer repair get sized once for the whole property. A common Valley plan is a garage conversion ADU now and a second-story addition later, once the rent is coming in; another is an attached ADU built as the parents’ suite with a detached rental behind it.

Where we do this work

The rules that decide between an ADU and an addition change with the address. Inside the City of Los Angeles and most of the San Fernando Valley, the 45 percent floor-area cap and the R1 setbacks apply to additions and LADBS reviews both projects, with the standard plan catalogue available for detached ADUs. In Orange County, each city sets its own floor-area and setback rules, and master-planned communities in Irvine and Anaheim Hills add HOA review, though state law voids covenants that prohibit or unreasonably restrict an ADU. Ventura County cities such as Thousand Oaks have their own building divisions and hillside lots that often need a soils report. In western Riverside County and western San Bernardino County, larger lots in Corona, Ontario and Rancho Cucamonga make a detached ADU and a ground-floor addition both feasible, and expansive soils can call for a geotechnical report. See all of our service areas.

ADU vs. room addition FAQs

Is an ADU cheaper than a room addition?

Per square foot they are close in Los Angeles: a ground-floor room addition typically runs $300 to $500 per square foot all-in and a detached ADU $300 to $450, while a second-story addition runs $400 to $650. In total an ADU often costs more than an addition of the same size because it always includes a kitchen and a full bathroom, and a garage conversion ADU at $100,000 to $200,000 is usually the lowest-cost route to a separate unit. These are typical Los Angeles budgeting estimates, not a price list or a quote; a real project can come in lower or higher.

Can I build an ADU if my lot is at its floor-area limit?

Yes. California Government Code sections 66321 and 66323 require a city to approve a detached ADU of up to 800 square feet with four-foot side and rear setbacks even where the lot’s floor-area ratio or lot coverage would block an addition of the same size. In the City of Los Angeles, a lot at the 45 percent Baseline Mansionization limit cannot add a room, but it can still build that ADU. Our ADU rules calculator shows what applies to your address.

Which adds more value, an ADU or an addition?

It depends on the street and the buyer. A room addition raises the value of the house toward the larger homes nearby; an ADU adds a separate dwelling that can produce rent and is marketed as a property with an income unit. Neither triggers a reassessment of your existing house; the county assessor adds only the value of the new construction. We do not publish value or rent figures because they vary by neighbourhood; an appraiser or agent who knows your street is the right source.

How long does an ADU take compared with a room addition?

An ADU in Los Angeles typically takes six months to a year from first meeting to move-in, and a room addition 9 to 14 months. The ADU is faster because state law requires the city to approve or deny a complete ADU application within 60 days and because construction does not disturb the occupied house, while plan check for an addition runs two to four months and a second-story addition can take five to seven months of construction alone.

Can I do a room addition and an ADU on the same lot?

Yes. The floor-area cap applies to the addition, and the state’s 800-square-foot ADU exemption applies to the ADU, so a homeowner can add a primary suite to the house and still build a detached unit in the yard, with a junior ADU of up to 500 square feet inside the house also allowed. Designing both on one set of drawings saves a second plan check and lets the panel upgrade and any sewer work be sized once for the whole property.

What is the difference between an attached ADU and a room addition?

An attached ADU is an addition with its own entrance, kitchen and bathroom, separated from the house by a wall, and it can be rented as a separate dwelling; a room addition opens into the existing house and is part of it. State law caps an attached ADU at 50 percent of the existing house’s floor area, with an 800-square-foot floor the city must allow, and lets it rise to 25 feet or the zone height limit. Homeowners who may want rental income later often choose the attached ADU and pay for the kitchen that makes it rentable.

Not sure whether your lot wants an ADU or an addition? Walk it with us.

We will check your setbacks and floor-area limit, camera the sewer, look at the panel and the garage, and tell you in writing which project fits your lot and your goal, with a budgeting range for each. The site visit and the estimate are free.