Building permits, zoning and septic rules in San Mateo County, California

Yes. Unincorporated San Mateo County requires a permit before a building or structure is erected, constructed, enlarged, altered, repaired, moved, improved, removed, converted or demolished¹. This mirrors state law: California requires a separate permit for each building or structure from the local building official before work starts, and code requirements vary by city and county¹⁰. The county's own guidance points applicants to pages 12–18 of its Building Ordinance to check whether specific work is exempt^1,11. California lets an owner build their own home without a contractor license under limited conditions — for example, a homeowner improving a principal residence must have lived there for the 12 months before completion, and cannot have used the exemption on more than two structures in a three-year period — but this only removes the licensing requirement, not the permit^6,12. If a project also needs a planning permit, the planning permit must be obtained before you apply for the building permit, and Design Review, Coastal Zone, Resource Management, Planned Agriculture and hazard-overlay districts always require planning review first¹. The county's rules apply to unincorporated land only. Inside Daly City, Redwood City and the Town of Hillsborough, the city issues its own building permits^13,14,15. Septic and well permits stay with County Environmental Health Services⁴. Farm buildings get one carve-out: an agricultural building (a non-residential structure for farm implements, hay, grain, livestock or greenhouses) is exempt from the automatic fire-sprinkler section of the county code, but not from building permits, and the local Fire District may still require sprinklers¹⁶.

The Planning and Building Department's Building Division reviews and inspects construction projects that require building permits in unincorporated San Mateo County, from an office at 455 County Center, 2nd Floor, Redwood City^11,17. Reach the building counter at (650) 599-7310 or buildingcounter@smcgov.org, and inspections at (650) 599-7311; the office is open Monday through Thursday, 8:00 am to 4:00 pm, closed for lunch from noon to 1:00 pm, and closed Friday through Sunday¹¹. The county's Building Service Fee Schedule took effect April 1, 2026, and a separate Affordable Housing Impact Fee applies to residential projects: a single-family home adds up to 2,500 square feet at no impact-fee charge, while projects over 3,500 square feet of net new floor area pay the fee on the full total¹⁸. Zoning questions go to the Planning Division at (650) 363-1825, in the same building¹¹.

Yes. The County Zoning Ordinance regulates growth and development across unincorporated San Mateo County, and the county's GIS layer labeled "Unincorporated SMC Zoning" shows a parcel's zoning district². Cities zone their own land: Daly City, Redwood City, Hillsborough, Woodside, Brisbane and Colma each run their own zoning, so a buyer near a city line should confirm with that city's planning department^{3,19,20,21,22,23,24}. A parcel can carry a base district plus overlays such as S (development standards), CD (coastal zone) or DR (design review), and must comply with each². One-family dwellings must be at least 18 feet wide at the narrowest elevation; the replacement ordinance is not yet effective in the coastal zone while it is under Coastal Commission review⁸. We could not confirm a countywide minimum floor area, so ask the Planning Division¹¹. In the Devonshire neighborhood's S-102 combining district, the code sets a 20,000-square-foot minimum building site, 20-foot front and rear setbacks, 10-foot side setbacks, 25% maximum site coverage and a 30-foot height limit; these are neighborhood-specific, not countywide⁸.

Septic systems are not handled by the building department — San Mateo County Environmental Health Services administers local oversight of onsite wastewater treatment systems (OWTS) under its Local Agency Management Program, and anyone installing an individual water supply or sewage disposal system needs a permit from Environmental Health first^4,25. Inspection staff observe and approve the work whenever a system is installed, repaired, altered or destroyed⁴. Statewide, California's OWTS Policy lets counties run their own approved local program as long as it stays consistent with state standards, which is what San Mateo County does²⁶. Soils matter here: only about 16.3% of San Mateo County's soils have no severe limitations for a conventional drain field, so most parcels will need Environmental Health's site evaluation before you can count on a standard system, and an alternative or engineered design is common⁵. A water-well drilling permit also comes from Environmental Health Services; it is non-transferable, authorizes only the recorded well driller, may first require a Coastal Development Permit or exemption determination from Planning and Building, and needs at least 2 full working days' notice before drilling begins⁴. Sewer connection is a separate track: inside a County-maintained Sewer District, a Sewer Inspection Permit, a sanitary sewer plan review or both may be required²⁷. Where there is no municipal sewer, the county states plainly that a septic system is the only approved way to dispose of sewage and wastewater on your own property⁴.

Permanent RV living is not generally allowed in California: RVs are intended for temporary occupancy and ordinarily may be occupied only in mobilehome parks or special occupancy parks unless a local ordinance provides otherwise²⁸. San Mateo County's Chapter 8.20 district does provide a limited construction exception: a mobilehome, travel trailer or RV may serve as temporary living quarters on the same parcel while a permanent dwelling is built or reconstructed, without a planning permit, but it must be removed before the permanent home's Certificate of Occupancy is issued⁹. That exception is district-specific, so confirm the parcel's zoning with the Planning Division before moving an RV onto the land^9,11. A tiny home must qualify under an approved building or vehicle classification and be placed where that classification is allowed; otherwise it cannot legally be occupied²⁸. San Mateo County's Farm Labor Housing Loan Program recognizes a tiny home as one possible housing unit, but only for participating rental projects serving eligible very-low-income farm laborers and their families; those projects still must meet Planning and Building, Environmental Health and permit requirements²⁹. The county's RV sites at Coyote Point Recreation Area are camping, not residence: the 4 designated sites are open March 1 through October 31, require reservations, cap a visit at 7 days within any four-week period, and limit RVs to 36 feet³⁰.

1. Confirm the parcel is unincorporated and pull its zoning district(s) through the county's GIS zoning layer^2,11.
2. If the site will use a well or septic, contact Environmental Health Services early for a site evaluation^4,25.
3. If the parcel is in a FEMA flood zone other than Zone X, plan for an Elevation Certificate at plan check, at framing inspection, and before occupancy³¹.
4. Submit the building permit application to the Building Division, filing an owner-builder declaration if you're not hiring a licensed contractor^6,11,12.
5. Pay the Building Service Fee Schedule charges and, for larger homes, the Affordable Housing Impact Fee¹⁸.
6. Complete plan review and pass the required inspections, ending with a Final Inspection before occupancy^11,17.
7. If you're building or altering a driveway onto a county road, get an encroachment permit from the Department of Public Works⁷.

San Mateo County is expensive and densely settled: the typical home value is about $1,595,764, population density runs around 1,619 people per square mile, and the nearest city of 100,000 or more is roughly 11 miles from the county's center⁵. Only 4.6% of the county is federal land, and 51.6% of the land is at 15% slope or less, so terrain deserves a parcel-level review⁵. Just 16.3% of soils here have no severe limits for a conventional drain field, so check septic suitability before you fall for a parcel⁵. Ask the Floodplain Management team whether a parcel sits in a mapped flood zone, since anything but Zone X requires at least a foot of elevation above base flood³². Planning to split a parcel? Unincorporated land divisions fall under the County Subdivision Regulations; a minor subdivision of up to 4 parcels is listed at roughly 6–8 months and about $14,600³³. Planning to rent a coastal-zone home short-term? A permit is required, with a limit of 180 nights a year³⁴. The county gets about 28.7 inches of rain a year, with severe drought in roughly 31.9% of years⁵. Budget for an effective property tax rate near 0.6% and a median tax bill around $9,167 a year⁵.