Yes. California law lets you convert a single-family home into two or more units in two main ways: (1) add a second primary unit under SB 9 (Gov. Code § 65852.21 and § 66411.7), and (2) add accessory units under State ADU Law (Gov. Code § 66323 and JADUs under § 66333). SB 9 does not require cities to allow more than four units on a single lot in any combination of primary units and ADUs/JADUs.
SB 9: creating a duplex
- SB 9 authorizes a second primary dwelling on a single-family lot (two‑unit development), enabling a duplex. While SB 9 and ADU law can be combined, SB 9’s cap means a local agency is not required to allow more than four units on a single lot total (e.g., two primary units plus up to two ADUs) under § 65852.21 and § 66411.7.
ADU/JADU on a single-family lot
- One detached, new‑construction ADU is allowed on a lot with an existing or proposed single‑family home, up to 800 square feet; objective side and rear setbacks may be no more than 4 feet (Gov. Code § 66323 with size tied to § 66321(b)(4); setbacks under § 66314(d)(7)).
- In addition, one conversion ADU may be created from existing space, and one JADU may be created within the single‑family residence (Gov. Code § 66323(a)(1); § 66333).
- ADU/JADU applications are subject to a streamlined clock: if a completed application is not approved or denied within 60 days, it is deemed approved (Gov. Code § 66317(a)).
After it’s a duplex: adding more units
- For ADU purposes, a building with two or more attached dwellings is a multifamily structure. Once you have a duplex, multifamily ADU rules apply: at least one conversion ADU is allowed within non‑livable areas, up to 25 percent of the number of existing units; and detached ADUs are allowed—up to eight total but not to exceed the number of existing units on the lot (Gov. Code § 66323(a)(3)–(4)), while JADUs are not permitted on multifamily sites (Gov. Code § 66333(a)).
What this does not address
- The source text does not list SB 9 eligibility criteria (e.g., site‑specific qualifications, exclusions), local objective standards that may apply to SB 9 projects, or any coastal/historic constraints for SB 9 two‑unit developments. It also does not provide local zoning nuances beyond the statewide ADU and SB 9 caps and standards cited above.