The short answer
If you own a duplex or an apartment building, state law lets you add ADUs two ways, and combine them: convert unused space in the building into units (at least one, up to 25% of the existing units), and build detached ADUs on the lot (up to eight, never more than the existing units).
Converting space inside the building
- Space not used as living space, such as storage rooms, boiler rooms, passageways, attics, basements or garages, can become ADUs.
- A city must allow at least one, and up to 25% of the existing units (Government Code § 66323(a)(3)).
Detached ADUs on the lot
- On a lot with an existing multifamily building: up to eight detached ADUs, and no more than the number of existing units (§ 66323(a)(4)).
- With a proposed multifamily building: up to two.
- Side and rear setbacks can't be more than 4 feet, and the city can't make you change an existing building whose setbacks are smaller.
What counts, and parking
- A multifamily building means two or more attached homes on one lot; several detached houses on one lot don't count (ADU Handbook).
- Garages, carports and uncovered spaces removed to build or convert an ADU don't have to be replaced (§ 66314(d)(11)).
- Cities can't add their own design standards to these state-law ADUs beyond what § 66323 allows.
Where cities differ
Some local codes still limit multifamily ADUs more than state law does; where they conflict, state law controls. Each city page's rules table shows its code and its standing with the state.
General information, not legal advice. Check the building's rent-control status and your city's requirements before you design.
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General information, not legal, zoning or financial advice. Rules change
and depend on your lot; check with your city before you design or build.