Accessory Dwelling Unit (ADU) Rules for Unincorporated San Mateo, CA

ADU State Law

What can you build in California?

• –Cannot bar a 150 sq ft Efficiency Unit.
• –Local caps may not be lower than 850 sf (1 BR) or 1 000 sf (2+ BR).
• –must always allow one 800 sf detached ADU with By-Right Permitting – Every city and county must process ADU and JADU applications ministerially; no public hearing or CEQA review.
• –once the application is deemed complete, the jurisdiction has 60 calendar days to issue or deny or it's automatically approved.
• –Single-family lots: at least 1 internal/attached ADU + 1 JADU + 1 detached ADU.
• –Multifamily lots: ≥ 1 (up to 25 % of existing units) made from interior conversions and 2 detached ADUs (existing buildings) or up to 8 detached ADUs total (SB 1211, 2025).
• –ADU (Accessory Dwelling Unit): may be detached, attached, or internal; fully independent with its own bathroom and full kitchen; no owner-occupancy requirement.
• –JADU (Junior ADU): max 500 sf, carved from within the existing single-family home; kitchenette allowed and bathroom may be shared; owner of the property must occupy either the JADU or the main house.
• –4-ft side/rear setbacks.
• –16 ft base; 18 ft within ½-mile of a Major Transit Stop/HQTC or on a lot with a multistory house; +2 ft to match roof pitch (max 20 ft).
• –Side/rear setbacks can't exceed 4 ft; front-yard placement must be allowed somewhere on the lot if needed to fit the 800 sf default unit.
• –ADUs are legal in every residential zone.
• –Exempt from local lot-coverage, FAR, bedroom caps, aesthetic design rules, etc., if those rules would block the 800 sf / 4-ft-setback unit.
• –Cities may adopt friendlier standards but not stricter ones; if they adopt none, state rules apply by default.
• –Impact fees: forbidden for units < 750 sf; proportional for larger units.
• –Fire sprinklers: not required when the primary dwelling lacks them.
• –Connection fees / capacity charges: must be proportional to fixture counts.
• –No spaces required when the ADU is: • within ½-mile walk of transit • in an historic district • wholly inside the main building • in a car-share area • replacing a garage/carport being converted.
• –No replacement parking may be demanded for a garage conversion.
• –Owner-occupancy cannot be required for ADUs. It is still required for JADUs.
• –Cities may limit short-term rentals (< 30 days).
• –No statewide bedroom limit.
• –Cities that pass an ordinance under AB 1033 may allow the ADU to be sold separately as a condo; otherwise ADU and main home must remain on one lot.
• –Home-owner associations cannot unreasonably bar ADUs/JADUs (Civil Code § 4751).
• –Building-code upgrades (Chapter 7A) apply in mapped Fire-Hazard Severity Zones and the Wildland–Urban Interface.
• –State ADU rights still apply, but a Coastal Development Permit is needed unless the project is an interior conversion with no added floor area.
• –After a lot is split, each new parcel is limited to two total units (e.g., primary + ADU/JADU), not the full stack above.
• –By Jan 1 2025 every city and county must publish a free library of pre-approved ADU plan sets (AB 1332).
• –State ADU law doesn't override private park rules or recorded easements; check title/park covenants case-by-case.