A granny flat is a small, self-contained home on the same lot as a house. In California it is permitted as an accessory dwelling unit; “granny flat” is everyday language and appears nowhere in state law. The useful question is not what to call it but what will actually fit behind your house — and state law answers a surprising amount of that for you. Every California city must allow a detached unit of at least 800 square feet with four-foot side and rear setbacks, and must approve or deny a complete application within 60 days.
Green Design and Build is a licensed general contractor (CSLB #1110975) based in Van Nuys. We design, permit and build ADUs across Los Angeles, Orange and Ventura counties and the western Inland Empire. This page is about feasibility: what the law guarantees you, what your lot decides, and how to check both before you spend money on drawings.
Is a granny flat a real category in California?
No — and that is good news. “Granny flat” is British and Australian in origin and arrived in California as shorthand for a small unit built for an ageing parent. It has no definition in the Government Code and none in the Los Angeles Municipal Code, so nobody is going to refuse your permit over the word.
What your building department will decide is which of three things you are building:
- An ADU — a separate home with its own kitchen, bathroom and entrance, whether detached, attached, or converted from a garage.
- A junior ADU — up to 500 sq ft inside the walls of the existing house, with an efficiency kitchen and its own exterior door.
- An accessory structure — no kitchen, so nobody may live in it as an independent home.
Almost everyone who says “granny flat” means the first one. Our guide to what an ADU is and what all the other names mean covers the full vocabulary; this page is about whether one fits.
What state law guarantees you
California ADU law works by setting floors that no city ordinance may go below. These are the ones that decide feasibility on a small lot.
| Provision | The state floor | Why it matters on a tight lot |
|---|---|---|
| Size a city must allow | At least 800 sq ft of interior livable space with four-foot side and rear setbacks | Lot coverage, floor area ratio, open space, front setback and minimum lot size rules cannot be used to block this |
| Maximum size a city may impose | No lower than 850 sq ft, or 1,000 sq ft with more than one bedroom | The 1,200 sq ft you see quoted is a local ceiling, not a state one |
| Minimum size | A city may not set a minimum that prohibits an efficiency unit | Very small units are legal in principle |
| Height, detached | At least 16 ft; at least 18 ft within half a mile walking distance of a major transit stop or high-quality transit corridor, plus two more feet to match the main house’s roof pitch | Decides whether a loft or a taller ceiling is possible |
| Height, attached | 25 ft, or the limit that applies to the main house, whichever is lower | Relevant to attached additions rather than backyard units |
| Impact fees | None at 750 sq ft or less; proportionate above | 750 sq ft is a genuine cost cliff worth designing around |
| Parking | No replacement parking required when a garage is converted | Removes the most common objection to garage conversions |
| Fire sprinklers | Not required in the ADU if the existing house has none | Avoids a significant cost and often a water service upgrade |
How small can a granny flat be?
Smaller than most people assume. California’s Health and Safety Code permits an efficiency unit with a minimum floor area of 150 square feet for up to two occupants, and state ADU law separately forbids a city from setting a minimum that would prohibit an efficiency unit. Underneath that sit the ordinary residential code minimums: a habitable room must be at least 70 square feet and at least 7 feet in any horizontal dimension, with ceilings at least 7 feet high — 6 feet 8 inches in a bathroom or laundry.
You will see 190 square feet quoted as the minimum on a great many contractor websites. There is no statute behind that figure. Build small deliberately if the lot demands it, but build it to the real numbers.
How to check your own lot in an afternoon
- Get your lot dimensions. The county assessor’s parcel record gives you the lot size and shape. A rectangular lot makes this easy; a flag lot, a corner lot or a hillside lot rarely does.
- Measure from the back of the house to the rear property line, and from the house to each side line. A tape measure and an hour is enough at this stage. Four feet of that belongs to the setback on the side and rear.
- Subtract what is already spoken for. The driveway you still need, the sewer lateral, the main water line, the electrical service run, any protected tree and its root zone, and any easement across the back of the lot. Easements are the one that catches people, and they are on the title report you already have.
- Read your own city’s ordinance, because the state floor is a floor. Our Los Angeles ADU rules by city page and the ADU rules calculator are the quickest way in for a specific address.
If what is left is a rectangle of roughly 20 by 30 feet, you have room for a comfortable one-bedroom. If it is narrower, a long, shallow plan along the rear fence often works better than trying to centre a square building. If there is genuinely no room, a garage conversion or a junior ADU inside the house is usually the answer rather than the end of the idea — we compare them in detached ADU vs. garage conversion.
Pre-approved designs can shorten everything that follows. The city’s standard plan programme offers vetted detached plans across a wide range of sizes; our page on ADU standard plans in Los Angeles explains how they work.
The 60-day clock, and what “deemed approved” means
This is the part of California ADU law homeowners most often do not know they have. Under Government Code section 66317, an ADU permit application is decided ministerially — no discretionary review, no hearing, no neighbours objecting at a public meeting. And the clock is real:
- The agency must decide whether your application is complete within the statutory period and, if it is not, give you a written list of what is missing and how to fix it. It may not later demand something that was not on that list.
- If it fails to make that completeness determination on time, the application is deemed complete.
- Once complete, the agency must approve or deny within 60 days where there is already a house on the lot.
- If it does neither within 60 days, the application is deemed approved.
- A denial has to come back in writing, within that same period, with a full set of comments.
Two practical consequences. First, a complete, well-prepared submittal is worth real money, because the clock only starts when the application is complete — most delay is caused at the front end, not by the city. Second, if you are being told informally that “it will take as long as it takes,” that is not what the statute says. Note that the 60 days is tolled if you request a delay, and runs differently when the ADU is submitted alongside an application for a brand-new house on the same lot.
Permit approval is not the whole schedule, of course — design, engineering and construction still take the time they take. Our page on how long an ADU takes to build sets out the realistic end-to-end picture, which is usually six months to a year.
What a granny flat costs in Los Angeles
| Route | Typical size | Typical Los Angeles budgeting range |
|---|---|---|
| Junior ADU inside the house | Up to 500 sq ft | $60,000–$150,000 |
| Garage conversion | The existing garage footprint, commonly about 400 sq ft | $100,000–$200,000 |
| New detached granny flat | About 500 sq ft | $180,000–$260,000 |
| New detached granny flat | 700–800 sq ft | $225,000–$350,000 |
| New detached granny flat | About 1,000 sq ft | $300,000–$450,000 |
The costs that catch people out are rarely the finishes. They are the electrical service upgrade an older house needs before it can carry a second unit, the sewer lateral if a camera finds it broken, the trenching distance from the house to the new building, and the site work on a sloped lot. Ask about all four before you compare two quotes — most of the variation between bids comes from here, as we explain in why ADU quotes vary so much. Financing routes are covered on our ADU financing options page.
Does a granny flat add value?
It adds a second legal dwelling and rentable square footage, and in Los Angeles County the rent a small unit commands is substantial — we put current HUD rent benchmarks against build cost on our ADU rental income page.
Resale value is a separate question, and anyone quoting you a confident percentage is guessing. What an ADU adds at sale depends on the appraisal, and appraisers in many neighbourhoods still have few comparable sales of houses with permitted ADUs to work from. Two things you control do matter: the unit must be permitted and finalled, because an unpermitted conversion is a liability at sale rather than an asset, and the paperwork — permits, final inspection, plans — should be kept somewhere you can find it years later. Build it legally and document it, and you have an asset a buyer, a lender and an appraiser can all see.
Building new living space is also new construction for property tax purposes: the county assessor adds the assessed value of what you built to your existing assessment, and your original home is not reassessed.
Mistakes we see on small lots
- Designing before checking the easements. A utility easement across the rear of the lot can make the obvious location the one place you cannot build.
- Crossing 750 sq ft without meaning to. Going from 740 to 780 sq ft changes the impact fee position for forty square feet of floor.
- Ignoring the trench. Water, sewer and power have to reach the new building. A unit at the far back corner of a deep lot costs more than the same unit close to the house, before a single fixture is chosen.
- Assuming a converted garage has a usable slab. Garage slabs are often sloped to drain and frequently have no vapour barrier. Both are solvable; neither is free.
- Submitting an incomplete application. The 60-day clock starts when the application is complete, so an incomplete submittal costs you the thing the statute was meant to give you.
- Legalising an old conversion the wrong way. A unit built without permits before 2020 follows a different path from converting a lawfully permitted structure today. Get it classified correctly first.
Granny flat FAQs
What is a granny flat?
A granny flat is a small self-contained home on the same lot as a main house, with its own kitchen, bathroom and entrance. The term is informal. In California the same building is permitted as an accessory dwelling unit, or as a junior accessory dwelling unit if it is built inside the walls of the existing house.
Are granny flats legal in California?
Yes. State law requires every city and county to allow accessory dwelling units on lots with an existing or proposed home, and applications are approved ministerially rather than through a hearing. Local ordinances add detail on top of the state minimums, but they cannot go below them.
How big can a granny flat be in California?
A city may not cap an ADU below 850 square feet, or below 1,000 square feet for a unit with more than one bedroom, and must allow at least 800 square feet with four-foot side and rear setbacks whatever its other development standards say. There is no statewide maximum — the 1,200 square feet quoted on most websites is a local ceiling, and a city may allow more.
How small can a granny flat 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 impose a minimum size that prohibits one. Residential code minimums still apply: habitable rooms of at least 70 square feet, at least 7 feet in any horizontal dimension, with 7-foot ceilings.
How close to the property line can I build?
State law requires cities to permit a detached ADU with four-foot side and rear setbacks. Front setbacks are set locally, and a city cannot use a front setback requirement to prevent an 800 square foot unit with four-foot side and rear setbacks from being built.
How long does the permit take?
Once your application is complete, the agency has 60 days to approve or deny it where a house already exists on the lot, and if it does neither the application is deemed approved. Getting to a complete application — design, engineering, the required documents — is the part that takes real time.
Can I rent out a granny flat?
Yes, on a lease of 30 days or longer. Short-term rental is not permitted, and cities may be stricter. An ADU cannot be sold separately from the main house on a single-family lot.
Do I need parking for a granny flat?
You are not required to replace parking lost when a garage is converted into an ADU. Parking requirements for new units are limited by state law and vary by location, particularly near transit, so check your city’s ordinance for your address.
Find out what fits on your lot
The fastest way to an answer is someone standing in your backyard with a tape measure and the ordinance for your city. We will check setbacks, utilities, access and slope, tell you what size unit is realistic at your address, and put a written budgeting range against it. Site visits take about two hours and a written proposal follows within 48 hours.