At a Los Angeles baseline in 2026, a garage conversion runs about $200–$350 per square foot, an attached ADU $300–$500, and a detached custom ADU $400–$700 — so a typical 800–1,200 sq ft detached unit lands around $320k–$840k before soft costs. California law makes ADUs allowed on most single-family lots: up to 1,200 sq ft detached, setbacks of no more than 4 ft at the side and rear, no owner-occupancy requirement, and approval within 60 days of a complete application. Plan on 10–18 months from first drawing to move-in. The rules make an ADU easy to permit; they do nothing to make it good to live in. That part is design.
Four kinds of ADU
“ADU” covers four quite different projects, and the first decision is which one you are actually building. Each has its own rules, cost and timeline.
| Type | What it is | Typical size | Cost at an LA baseline |
|---|---|---|---|
| Garage conversion | An existing garage or accessory structure turned into a dwelling | 300–500 sq ft | $200–$350 / sq ft |
| Attached ADU | An addition to the main house with its own entrance, kitchen and bath | 500–1,000 sq ft | $300–$500 / sq ft |
| Detached ADU | A new, separate building — the backyard cottage or guest house | 600–1,200 sq ft | $400–$700 / sq ft |
| Junior ADU (JADU) | Up to 500 sq ft carved out of the existing house, with an efficiency kitchen | Up to 500 sq ft | Usually the least expensive route |
A single-family lot can usually have at least one ADU and one JADU, and in some combinations more. Conversions are the cheapest and fastest; detached units are the most flexible and the most expensive, and they are where design makes the biggest difference to how the property lives.
What it costs
These are the same per-square-foot baselines our cost calculator uses for Los Angeles, before it applies a city index:
- Garage conversion: $200–$350 / sq ft. Cheapest because the structure, slab and roof exist. Watch for slabs that need replacing, inadequate foundations and electrical service that has to be upgraded.
- Attached ADU: $300–$500 / sq ft. Shares a wall with the house, which saves on envelope but means tying into existing structure, roof and systems.
- Detached custom ADU: $400–$700 / sq ft. A small house, with all of a house’s fixed costs — foundation, roof, kitchen, bathroom, mechanical, utility runs — spread over fewer square feet. That is why ADUs cost more per foot than the main house often did.
On top of construction, budget for soft costs: design and engineering, survey, permit and plan-check fees, utility connections, and in some cases school fees on larger units. State law exempts ADUs under 750 sq ft from impact fees, and limits them proportionally above that. For how design fees are structured, see design fees and budget ranges; for the full picture of costs beyond construction, see soft costs on a luxury build.
The biggest swing factors are rarely the finishes. They are site access (can a truck reach the back of the lot?), utility distances from the street, slope, and whether the sewer line has to be upsized or relocated.
What California allows
State law sets a floor that cities cannot go below, which is why ADUs are now buildable on most residential lots in California. The headline provisions:
- Where: on lots zoned for residential use with an existing or proposed single-family or multifamily dwelling.
- Size: a detached ADU can be up to 1,200 sq ft. A city cannot cap ADUs below 850 sq ft (studio or one bedroom) or 1,000 sq ft (two or more bedrooms). An attached ADU may be limited to 50% of the main house, but an 800 sq ft unit must still be allowed.
- Setbacks: no more than 4 ft at the side and rear for a new ADU, and none for a conversion of an existing structure.
- Height: 16 ft for a detached ADU, with more allowed near major transit and for attached units.
- Approval: ministerial — no discretionary hearing — with a decision required within 60 days of a complete application.
- Owner occupancy: not required for an ADU. A JADU still requires that the owner live in either the JADU or the main house.
- Parking: often not required — for example within half a mile of public transit, or when the ADU is created by converting existing space. Converting a garage does not require you to replace its parking.
Two things the state rules do not override: the California Coastal Act, so coastal-zone properties may still need a coastal permit or exemption (since October 2025, AB 462 requires a decision on a coastal permit for an ADU within 60 days of a complete application), and genuine fire and life-safety requirements. Details change with each legislative session, so confirm current standards for your parcel before you design to them.
Los Angeles specifics
“Los Angeles” is several dozen jurisdictions, and each applies the state framework through its own ordinance. The ones that matter most in practice:
- City of Los Angeles runs a large ADU program and offers pre-approved standard plans, which can shorten plan check for a simple unit. Hillside lots, and especially lots in Very High Fire Hazard Severity Zones, can face additional access and fire requirements — on some, only a conversion of existing space is practical. Check this first.
- Coastal cities and neighborhoods — Santa Monica, Malibu, parts of Pacific Palisades and Manhattan Beach — layer coastal review on top. See how coastal permits work.
- Fire rebuild properties in the Palisades and Altadena often add an ADU as part of the rebuild; sequencing it with the main house matters. See our fire rebuild resource centre.
- Independent cities such as Beverly Hills, Pasadena and Calabasas each have their own design standards for materials, height and placement within the state limits.
How long it takes
- Feasibility and design: 2–4 months. Survey, what the lot allows, utility routes, layout, materials and drawings.
- Permits: 2–6 months. The 60-day clock runs from a complete application; plan-check corrections, structural review, utility sign-offs and any coastal or fire review add to it.
- Construction: 5–9 months for a detached unit, less for a garage conversion.
Realistically that is 10–18 months from first drawing to move-in for a detached ADU. Designing it together with a main-house remodel usually saves time overall, because survey, engineering and permits run once.
Designing one that is worth it
The rules guarantee you can build an ADU. Whether it adds real value — as a rental, a guest house, a home office, a place for parents or grown children — comes down to decisions the rules say nothing about:
- Placement and privacy. Where the ADU sits decides how the whole garden works and whether two households overlook each other. On our Santa Monica bungalow, the detached ADU sits at the back of the lot with a dining pergola, and a courtyard links it to the house rather than dividing the yard.
- Light in a small footprint. Full-height glass, clerestories and skylights do more for a 600 sq ft unit than any finish.
- Storage and a real kitchen. These are what make a small unit livable for a long stay instead of a weekend.
- One design language. An ADU designed in the same model as the main house — materials, rooflines, lighting — reads as part of the property, not something dropped into the yard. Our Brentwood ADU and North Hollywood backyard and ADU are both examples.
- Walk it before you build it. In a small space, a few inches matter. We model every ADU in 3D and walk the owners and the contractor through it in a 360° walkthrough before construction, so the layout is settled before anything is poured.
Common mistakes
- Starting with a floor plan instead of the site. Utility runs, access and slope decide what is affordable; the plan comes after.
- Assuming the state rules settle everything. Coastal zones, fire zones, easements, protected trees and sewer capacity can all change what is practical.
- Designing the ADU separately from the house. It costs more in permits and coordination, and the result rarely looks like one property.
- Underbuilding the kitchen and storage to save a little, then finding the unit is hard to rent or live in.
- Choosing a unit that is too large for the lot. The largest allowed ADU is not always the one that leaves you a usable garden.
Common follow-up questions
How much does an ADU cost in Los Angeles?
At a Los Angeles baseline in 2026, a garage conversion runs about $200-$350 per square foot, an attached ADU $300-$500, and a detached custom ADU $400-$700. A typical 800-1,200 sq ft detached unit lands around $320k-$840k before soft costs such as design, engineering, permits and utility connections. Site access, utility distances and slope move the number more than finishes do.
How big can an ADU be in California?
A detached ADU can be up to 1,200 sq ft under state law, and a city cannot cap ADUs below 850 sq ft for a studio or one bedroom, or 1,000 sq ft for two or more bedrooms. An attached ADU can be limited to 50% of the main house, but an 800 sq ft unit must still be allowed. A Junior ADU is limited to 500 sq ft inside the existing house.
Do I need to provide parking for an ADU?
Often not. Parking is not required in a range of situations, including within half a mile of public transit and when the ADU is created by converting existing space. Converting a garage into an ADU does not require you to replace the lost parking. Confirm the exemptions that apply to your parcel.
Do I have to live on the property to build an ADU?
No. California permanently removed the owner-occupancy requirement for ADUs. A Junior ADU is different: the owner must live in either the JADU or the main house.
Can I build an ADU in the coastal zone or a fire zone?
Usually, but with extra steps. State ADU law does not override the California Coastal Act, so properties in the coastal zone - much of Santa Monica, Malibu and parts of Pacific Palisades - may need a coastal permit or exemption. In the City of Los Angeles, hillside lots in Very High Fire Hazard Severity Zones can face additional access and fire requirements, and on some only a conversion of existing space is practical. Check this before designing.
How long does it take to permit and build an ADU in Los Angeles?
Plan on 10-18 months for a detached ADU: roughly 2-4 months for feasibility and design, 2-6 months for permits, and 5-9 months for construction. The 60-day state approval deadline runs from a complete application, so plan-check corrections and any coastal or fire review add to it. Garage conversions are faster.
Will an ADU raise my property taxes?
Building an ADU is new construction, so the county reassesses the added value of the ADU. The existing house keeps its current assessed value; the ADU's value is added on top.
Can I sell an ADU separately or rent it short-term?
In most places an ADU cannot be sold separately from the main house unless the city has adopted an ordinance allowing it. Many Los Angeles-area cities restrict renting an ADU for less than 30 days, so check local rules before planning short-term rental. Long-term rental is generally allowed.
Sources
- Accessory Dwelling Unit Handbook (updated 2025, with 2026 addendum) — California Department of Housing and Community Development (HCD)State ADU rules: sizes, JADU 500 sf, ministerial review
- California Government Code § 66314 (ADU development standards) — California Legislative Information (Legislative Counsel)1,200 sf detached cap, 50% attached limit, no separate sale
- Government Code, Title 7, Div. 1, Ch. 13, Article 2: Accessory Dwelling Unit Approvals (§§ 66314–66331) — California Legislative Information (Legislative Counsel)60-day approval clock and minimum size guarantees
- AB-462 Land use: accessory dwelling units (Ch. 491, Stats. 2025) — California Legislative InformationCoastal-zone ADU permit timelines (60-day CDP decision)
- Accessory Dwelling Unit (ADU) — Los Angeles Department of Building and Safety (LADBS)Los Angeles ADU permitting and standard plans
- Fire Hazard Severity Zones — CAL FIRE – Office of the State Fire MarshalVery High Fire Hazard Severity Zone designations
Sources support the regulations, programs and public data discussed. Cost, fee and timeline ranges are Vividly Built's own planning estimates from our project work.