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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.