ADU rules in San Mateo, CA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
San Mateo allows ADUs in all residential zones (SMMC Ch. 27.19). One-bedroom ADUs up to 850 sq ft; two-bedroom up to 1,000 sq ft; detached max 16 ft plate/24 ft peak; 4-ft setbacks. ADUs cannot be STRs.
San Mateo Municipal Code Chapter 27.19 governs ADUs. Detached ADUs are limited to 16 feet to the top of the plate line (24 feet to roof peak). Attached ADU conversions may reach 24 feet to the plate line (32 feet peak). One-bedroom detached ADUs up to 850 sq ft; two-bedroom up to 1,000 sq ft. JADUs limited to 500 sq ft within existing structure. Setbacks: 4 feet from rear and interior side property lines. Windows within 5 feet of a lot line require obscured glazing. ADUs cannot be used as short-term rentals under SMMC Ch. 5.66. Permit review within 60 days; typical process 3–4 months.
Unpermitted ADUs subject to code enforcement and fines.
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Side-by-side rule comparisons with other cities in San Mateo County.
See how other cities in San Mateo County handle adu rules.
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