Lot coverage limits in unincorporated San Mateo County range from 25% to 50% depending on paired S-District and zoning. Typical R-1 with S-100-series: 40-45% maximum coverage. Hillside (RH): 25-30% with additional slope-based reductions. Coverage includes the footprint of all buildings and covered structures; impervious surface limits (including driveways and patios) separately apply under the NPDES Stormwater requirements (C.3) for projects creating or replacing 2,500+ sq ft of impervious surface.
Lot coverage, defined under County Zoning Regulations as the horizontal area of a lot covered by buildings including covered porches/decks and attached structures, is capped by the S-District standard. Typical values: S-100 (5,000 sq ft R-1): 45%; S-101 (7,500 sq ft R-1): 40%; S-17 (20,000 sq ft R-1 or RH): 30%; agricultural: 10-15%. Impervious surface (a separate calculation) includes buildings, driveways, patios, pools, and compacted gravel — projects creating or replacing 2,500+ sq ft of impervious surface trigger NPDES C.3 stormwater treatment requirements under the Bay Area Regional Municipal Stormwater Permit (Order R2-2022-0018). Permeable pavers may receive partial credit (typically 50%) toward impervious surface calculations but count fully toward lot coverage if they form a structural surface. Hillside lots with slopes over 20% have reduced allowed coverage. Exceeding coverage requires a variance through Planning Commission. ADUs under 800 sq ft are exempt from lot coverage calculations under state law.
Exceeding lot coverage: building permit denial, required redesign. Completed violations: correction (removal of excess structure) or variance with public hearing. Fines $200-$1,000 per §1.04. Impervious exceedance without C.3 treatment: stormwater violation $500-$10,000.
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See how San Mateo County's lot coverage limits rules stack up against other locations.
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