Short answer: Two state-law “floors” apply. First, a city cannot set a minimum unit size that would bar an efficiency unit; the smallest ADU a city must allow is therefore 150 sq ft (HSC § 17958.1). Second, even with lot coverage, FAR, open space, or front setback rules, a city’s standards must still allow at least one ADU of 800 sq ft with 4‑ft side and rear setbacks to be built on a qualifying lot (§ 66321(b)(3)).
What a city may and may not require
- Local ordinances may set minimum and maximum ADU sizes, but they cannot adopt a minimum that prevents an efficiency unit; “an efficiency unit may be as small as 150 sq ft” (HSC § 17958.1).
- Any maximums a city sets must still allow at least 850 sq ft, or 1,000 sq ft if the ADU has more than one bedroom (§ 66321(b)(2)).
The guaranteed 800 sq ft ADU
- Cities cannot apply zoning or development controls (e.g., lot coverage, FAR, open space, front setbacks, minimum lot size, or size-percentage limits) in a way that fails to “permit at least an 800 sq ft accessory dwelling unit with four‑foot side and rear yard setbacks” built in compliance with other local standards (§ 66321(b)(3)).
Conversions aren’t subject to unit-size limits
- If you convert existing space (e.g., part of a house or an accessory structure) to an ADU, state law says that type of ADU “is not subject to unit size requirements” (§ 66323(a)(1)).
In practice: a city must allow very small ADUs down to 150 sq ft (it cannot set a higher minimum), and its development standards must also leave room for at least one 800 sq ft ADU with 4‑ft side/rear setbacks on eligible lots. The 850/1,000‑sq‑ft rule limits how small a city’s maximums can be, but does not stop homeowners from building smaller units if they wish (subject to the 150‑sq‑ft minimum).