Room Addition Permit Requirements in San Mateo County, CA
San Mateo County Planning and Building Department serves all UNINCORPORATED areas of San Mateo County, including the Midcoast communities of Montara (94037), Moss Beach (94038), El Granada (94018), Princeton, and Miramar. This record currently covers 94037 Montara only; other unincorporated Coastside ZIPs will be added in follow-up work. Incorporated cities (Daly City, San Mateo, Redwood City, Pacifica, Half Moon Bay, Belmont, Burlingame, Foster City, Menlo Park, etc.) run their own building departments — confirm parcel is unincorporated before applying here. CRITICAL Coastside overlay: all of 94037 is in the California Coastal Zone, so nearly any development (new structure, major addition, grading, demolition, some remodels) requires a Coastal Development Permit (CDP) from SMC Planning under the Midcoast Local Coastal Program (Midcoast LCP, approved by California Coastal Commission August 2012). Only narrow exemptions apply; appeals go to the Coastal Commission. California 2025 Building Standards Code (CRC/CBC/CFC) took effect statewide 2026-01-01; state code is enforceable regardless of whether the County has signed its local 2025 adoption ordinance (historical adoption was Ord. 4873 on 2022 cycle). Portions of Montara (upslope toward Montara Mountain / McNee Ranch) are in CAL FIRE Very High Fire Hazard Severity Zones; 2025 CRC Chapter 7A (ignition-resistant construction) applies. Seismic Design Category D; ultimate design wind ~110 mph. Parcels use septic (SMC Environmental Health Services, LAMP program) and private wells unless on Montara Water & Sanitary District (MWSD, 650-728-3358) sewer+water service. Online permits via Accela Citizen Access (ACA) at aca-prod.accela.com/smcgov. Fee schedules effective 2026-04-01. Building counter at 455 County Center, Redwood City.
By Joey, Founder · Last verified April 24, 2026 · How we research →
When you need a Addition permit in San Mateo County
94037), any habitable-space addition requires a building permit, electrical and plumbing permits, AND a Coastal Development Permit. Plans must show structural (seismic Category D, 110 mph wind), Title 24 energy compliance, and Midcoast LCP view-corridor consistency. Timeline is longer than inland CA cities because of the dual CDP+building review.
Room Addition Permit Costs: San Mateo County vs. Nearby
| City | Estimated Fee |
|---|---|
| San Mateo County | N/A |
| Los Angeles | $1,000 – $20,000 |
| Sacramento County | $200 – $2,000 |
| San Diego | $850 – $17,000 |
Fees shown are estimates based on local building code data. Actual fees may vary based on project scope and valuation. See all San Mateo County permit costs →
Zoning & HOA Considerations
Room additions typically require zoning review for setbacks, height limits, and lot coverage. Your addition must comply with your zoning district's building envelope requirements.
If you live in an HOA community: room additions almost always require HOA architectural review approval. HOAs may restrict the size, style, materials, and exterior appearance of additions to maintain neighborhood consistency.
Estimated Project Timeline in San Mateo County
Here's a typical timeline for a room addition project in San Mateo County, CA:
Timeline is based on San Mateo County's typical about 11 weeks permit processing time. Actual timelines vary by project complexity, completeness of application, and current department workload.
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Frequently Asked Questions
Contact San Mateo County's building department for current Room Addition permit fee information. Fees typically vary based on project scope and valuation.
It depends on the specifics of your project. Some smaller Room Addition projects may be exempt from permit requirements, while larger ones typically require a permit. Use our free lookup tool to check your specific situation.
Working without a required permit in San Mateo County can result in fines, stop-work orders, being required to remove completed work, and complications when selling your home. It's always best to check first.
Typical approval time for Room Addition permits in San Mateo County is about 11 weeks. More complex projects may take longer if plan review is required.
If you live in an HOA community, you may need architectural review approval in addition to a building permit. HOA rules and city building codes are separate, you may need to satisfy both. Review your CC&Rs and submit to your HOA's architectural review committee before starting work.
Learn More About Room Addition Permits
Read our complete guide covering permit requirements, costs, common exemptions, and the application process.
Read Room Addition Guide →