San Mateo County, CA Building Setbacks & Zoning: Lot Coverage Limits (2026)

San Mateo County controls how much of a lot can be built upon through both 'building site coverage' (lot coverage) and floor-area ratio (FAR), set by the applicable district. In the Emerald Lake Hills district, Section 8.110.070 provides that not more than 25 percent of the building site may be covered by buildings, accessory buildings, or structures such as patios, decks, balconies, and porches that are 18 inches or more above the ground; Section 8.110.090 limits total floor area of all stories to 30 percent of the building-site area or 2,400 square feet, whichever is greater. The Executive/General district standards similarly use a 0.25 (25 percent) maximum building-site coverage ratio that counts buildings, accessory buildings, and structures, with a 20,000-square-foot minimum site in that context. Some districts and overlays are more restrictive: the COSC district (Section 8.48.010) permits only 10 percent lot coverage with a 2-acre minimum site and a 16-foot single-story height limit, while the Midcoast area limits impervious surface to roughly 10 percent of parcel size and sets parcel-size-based maximum floor areas. Coverage and FAR therefore vary widely depending on the parcel's base district, combining 'S' district, and any Local Coastal Program or design-review overlay. Because the exact percentage depends on the specific zoning, property owners should confirm the applicable lot coverage and floor-area limits for their parcel with the Planning and Building Department before designing an addition or new structure.