Accessory Structures in San Mateo, CA: What Residents Actually Need to Know

If you live in San Mateo or are thinking about moving there, accessory structures are one of those things you probably won't think about until they affect you directly. San Mateo has 5 specific rules on the books covering different aspects of accessory structures, and some of them might surprise you.

Shed Rules

Small accessory structures (sheds) in San Mateo must comply with SMMC zoning setbacks. Sheds under 120 sq ft and under 8 feet average height generally do not require a building permit but must comply with zoning requirements.

Key details: Small Sheds (<120 sq ft): Generally no building permit. Setbacks: Typically 4 ft from rear/side boundaries. Habitable Use: Requires ADU permit. Contact: Building Division (650) 522-7172.

Unpermitted structure: retroactive permit with penalty. Non-compliant setback: modification or removal. Habitation violation: immediate correction.

Garage Conversions

Garage conversions to ADUs are permitted in San Mateo per California ADU law. Building permit required. No replacement parking is required when converting a garage to an ADU under state law.

Key details: Permitted: Yes, per CA ADU law (SMMC Ch. 27.19). Building Permit: Required. Replacement Parking: Not required (state law). Contact: Planning (650) 522-7010.

Unpermitted conversion: standard building code enforcement. Must bring to code or restore to garage. Safety violations: immediate correction.

ADU Rules

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.

Key details: Max Size (1 BR): 850 sq ft. Max Size (2 BR): 1,000 sq ft. Detached Height: 16 ft plate / 24 ft peak. Setbacks: 4 ft rear and interior sides. STRs Prohibited: ADUs cannot be used as short-term rentals.

Unpermitted ADUs subject to code enforcement and fines.

Carport Rules

Carports in San Mateo require a building permit under Title 23 and must comply with the zoning standards in Title 27. Carports count toward lot coverage calculations. Garage or carport conversions to ADUs are specifically addressed under Chapter 27.19.

Key details: Permit Required: Yes — building permit (Title 23). Lot Coverage: Included in calculation. ADU Conversion: Permitted under Ch. 27.19. Parking Replacement: Not required for ADU conversion. Contact: Community Development — 650-522-7200.

Building a carport without a permit violates Title 23. Code enforcement may require demolition or retroactive permitting. Setback violations require a planning application for variance or exception.

Tiny Homes

Tiny homes in San Mateo are regulated through the ADU provisions of Chapter 27.19 and California Government Code 65852.2. Tiny homes on permanent foundations may qualify as ADUs (up to 1,200 sq ft detached) or JADUs (up to 500 sq ft). Tiny homes on wheels are classified as RVs and cannot serve as permanent housing.

Key details: Max Detached ADU: 1,200 sq ft; 16 ft plateline. JADU: Up to 500 sq ft within structure. Tiny Home on Wheels: RV classification — not housing. Setbacks: 4 ft side/rear for new detached. State Law: CA Gov Code §65852.2.

Unpermitted tiny homes used as dwellings face code enforcement. Tiny homes on wheels used as permanent housing violate zoning law. Violations may result in fines and orders to vacate.

The Bottom Line

San Mateo's accessory structures rules are a mixed bag. Some areas are strict, others are relaxed, and the details matter. The best approach is to check the specific rule that applies to your situation rather than assuming San Mateo is broadly strict or permissive.

All of the above reflects San Mateo's municipal code as of our last review. If you need specifics on fines, exemptions, or filing requirements, the detailed ordinance pages linked above have the full breakdown.