ADU rules in San Mateo County, CA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
Unincorporated San Mateo County allows detached ADUs up to 800 sq ft or 35% of primary up to 1,500 sq ft; attached ADUs up to 800 sq ft or 50% up to 1,500 sq ft. ADUs up to 26 ft tall. 4-ft rear/side setbacks. ADUs under 750 sq ft exempt from impact fees.
San Mateo County's ADU Ordinance (updated to comply with CA state law) allows: detached ADUs — 800 sq ft or 35% of primary residence floor area, whichever is greater, up to 1,500 sq ft maximum; attached ADUs — 800 sq ft or 50% of primary, up to 1,500 sq ft maximum; JADUs — 500 sq ft (plus 150 sq ft for ingress). Height: up to 26 feet for two-story detached ADUs. Setbacks: 4 feet from rear and interior side property lines. Parking: JADUs require none; ADUs may require 1 space (tandem allowed) unless within 0.5 mile of transit. ADUs under 750 sq ft are exempt from all impact fees. Short-term rentals of ADUs are prohibited. Coastal Zone ADUs require additional coastal development permit.
Unpermitted ADU construction subject to county code enforcement.
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See how San Mateo County's adu rules rules stack up against other locations.
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