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San Mateo
County
What You Can Build in San Mateo
Under California state law, most residential properties in San Mateo are eligible for significant development beyond their current use. Here's what the law allows:
Accessory Dwelling Unit (ADU)
California law (AB 68, AB 881, SB 897) allows homeowners in San Mateo to build an ADU up to 1,200 sq ft on most single-family and multi-family lots. No minimum lot size required. ADUs can be detached, attached, or converted from existing space (garage, basement).
Junior ADU (JADU)
In addition to an ADU, San Mateo homeowners can build a JADU of up to 500 sq ft within the existing footprint of the primary home. Owner occupancy required for either the JADU or primary unit.
SB 9 Duplex + Lot Split
Single-family homeowners in San Mateo can split their lot into two parcels and build a duplex on each — up to 4 units total. Lot must be in a single-family zone, not in a historic district, and each resulting parcel must be at least 1,200 sq ft (40% of original lot).
SB 1211 — Multiple ADUs
Effective 2025, SB 1211 allows up to 8 detached ADUs on multi-family properties in San Mateo, plus additional ADUs from garage/carport conversions. Single-family lots can have an ADU + JADU.
AB 1033 — ADU Condo Conversion
If San Mateo has opted in, homeowners can convert ADUs into separately sellable condominiums. This allows ADUs to be sold independently from the primary home, creating new homeownership opportunities.
Frequently Asked Questions
Can I build an ADU in San Mateo?
Yes. Under California state law, most residential properties in San Mateo are eligible for at least one ADU up to 1,200 sq ft, regardless of local zoning restrictions. The state has preempted local ordinances that unreasonably restrict ADU development.
How much does it cost to build an ADU in San Mateo?
ADU construction costs in San Mateo County typically range from $150,000 to $400,000+ depending on size, finishes, and site conditions. Prefab/factory-built ADUs can reduce costs to $100,000-$250,000. Garage conversions are generally the most affordable option at $50,000-$150,000.
Can I split my lot under SB 9 in San Mateo?
If your property is in a single-family zone, not in a historic district, and meets minimum size requirements, you can split your lot into two parcels under SB 9. Each resulting parcel must be at least 1,200 sq ft. You can then build up to two units on each parcel.
Do I need a permit to build an ADU in San Mateo?
Yes, you need a building permit from the City of San Mateo. However, cities are required to approve ADU applications ministerially (without discretionary review) within 60 days if the project meets state standards. Impact fees are waived for ADUs under 750 sq ft.