In California, a detached ADU can be up to 1,200 square feet, an attached ADU can be up to 1,200 square feet or 50% of the primary home’s floor area (whichever is smaller), and a junior ADU is capped at 500 square feet. Cities can set lower maximums, but state law guarantees you the right to build an ADU of at least 800 square feet on a single-family lot. So the practical answer to how big an ADU can be in California depends on the type of unit, your local ordinance, and the shape of your lot.
Maximum Size by ADU Type
California’s ADU law, codified in Government Code sections 66310 through 66342, sets different ceilings depending on the type of unit.
- Detached ADU. Up to 1,200 square feet of floor area, regardless of the primary home’s size.
- Attached ADU. Up to 50% of the primary dwelling’s existing floor area, with an absolute cap of 1,200 square feet. An 1,800-square-foot home allows a 900-square-foot attached ADU. A 3,000-square-foot home hits the 1,200 cap.
- Junior ADU (JADU). No more than 500 square feet. A JADU must be built entirely within the walls of an existing or proposed single-family home, including enclosed spaces like an attached garage.1California Legislative Information. California Government Code 65852.22 – Junior Accessory Dwelling Units
On the small end, the smallest an ADU can legally be is 150 square feet. That comes from California’s efficiency unit standard, and no local agency can set a minimum that prohibits an efficiency unit of that size.2California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
Minimum Sizes Your City Must Allow
Cities can cap ADU sizes below the state’s 1,200-square-foot ceiling, but state law guarantees minimum floors that no local government can go below:
- Studio or no bedroom: at least 800 square feet
- One bedroom: at least 850 square feet
- Two or more bedrooms: at least 1,000 square feet
These floors apply to both attached and detached ADUs. If your city caps detached ADUs at 900 square feet, you can still build a two-bedroom unit at 1,000 square feet because the state floor overrides the local cap for that bedroom count.2California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
How Local Rules Affect Size
California lets cities and counties adopt their own ADU ordinances, but only within the state’s framework. A local agency can set a maximum below 1,200 square feet for detached ADUs, but it cannot drop below the 800/850/1,000-square-foot minimums above. A city might cap your detached ADU at 1,000 square feet; it cannot cap it at 750.
Local lot-coverage limits and floor-area-ratio (FAR) rules can also constrain how large your ADU turns out. State law prohibits those requirements from blocking an ADU of at least 800 square feet with four-foot side and rear setbacks. Open-space and landscaping rules get the same treatment. If your lot is tight and a strict FAR would prevent any ADU from being built, the 800-square-foot floor still stands.2California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
Because these local variations meaningfully change what you’re allowed to build, check your city or county’s ADU ordinance before designing anything. Some jurisdictions allow the full 1,200 square feet; others exercise their authority to impose lower caps.
Height Limits
How tall your ADU can be depends on where it sits and what’s nearby.
- Detached ADU, standard: 16 feet maximum.
- Detached ADU near transit or on a multifamily lot: 18 feet, if the property is within a half-mile walking distance of a major transit stop or high-quality transit corridor, or if the lot already has a multistory, multifamily building. An additional two feet is allowed to match the roof pitch of the primary home.
- Attached ADU: 25 feet or the height of the primary dwelling, whichever is lower. Local agencies are not required to allow an attached ADU above two stories.2California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
Setbacks and How They Shape Buildable Size
For a newly built attached or detached ADU, state law caps side and rear setbacks at four feet. Your local agency can require up to four feet but not more.2California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
Front-yard setbacks follow local rules, but those rules cannot prevent you from building an ADU of at least 800 square feet. If your lot is shallow and the front setback would make an 800-square-foot ADU impossible, the ADU can encroach into the front setback.
Conversions of existing structures get more generous treatment. No setback is required at all when the ADU is built within an existing structure, or when a new structure is built in the same location and to the same dimensions as an existing one.2California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
Special Rules for Garage and Structure Conversions
Converting an existing garage, shed, or other detached structure into an ADU follows a more relaxed set of size rules than building from scratch.
A converted ADU is not subject to the standard unit-size requirements. If your existing detached garage is 400 square feet, you can convert it to an ADU even though 400 square feet falls below the 800-square-foot minimums that apply to new construction. You can also add up to 150 square feet to the structure for ingress and egress features like a stairwell or entryway.2California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
Objective design standards related to height, lot coverage, landscaping, and architectural style do not apply to conversions. Your city also cannot force you to replace lost parking spaces when a garage, carport, or uncovered parking area is demolished or converted for an ADU.2California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
How Many ADUs You Can Fit on One Lot
Size ceilings matter more once you realize California allows multiple ADUs on a single property. The count depends on the type of home already on the lot.
Single-Family Lots
On a lot with an existing or proposed single-family home, you can build up to three units: one ADU converted from existing space in the home or an accessory structure, one newly constructed detached ADU, and one JADU. All three can coexist on the same lot.3California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
Multifamily Lots
On a multifamily property, the allowances are larger. The local agency must permit at least one ADU converted from existing non-livable space within the building, and up to 25% of the existing unit count in conversions. Under SB 1211, which took effect January 1, 2025, up to eight detached ADUs are allowed on a lot with an existing multifamily building, as long as the number of ADUs does not exceed the number of existing units on the lot.3California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
How ADU Size Is Measured
ADU size is measured by gross floor area, which includes all habitable space: bedrooms, living areas, kitchens, and bathrooms. Non-habitable areas like garages, carports, and covered patios do not count unless they are converted into living space. A garage-to-ADU conversion counts the former garage space as part of the unit’s floor area once it becomes habitable.
Getting the measurements right matters. Your permit application will require professional architectural or design plans showing exact dimensions, and an ADU that looks like it fits on paper but exceeds the allowed square footage by a small margin can stall the permitting process. Measure carefully during the design phase rather than after construction begins.