San Mateo County, CA Building Setbacks & Zoning: Setback Rules (2026)

Unincorporated San Mateo County uses a dual-district zoning system: the Use District (R-1 One-Family Residential, R-2, RH Residential Hillside, RM Resource Management, A Agricultural, etc.) sets what you can build, while the paired S-District (Development Standards) sets dimensional rules. Typical S-100 (minimum 5,000 sq ft R-1 lot) standards: 20 ft front, 5 ft side, 15 ft rear setbacks, 45% lot coverage, 36 ft height. S-17 (20,000 sq ft lots, common in Emerald Hills): 25 ft front, 10 ft sides, 20 ft rear, 35% coverage. Corner lots typically require the street-side setback to match the front setback. Resource Management and hillside districts often require larger setbacks, habitat buffers, and slope-stability reviews. Eaves may encroach up to 30 inches into a setback; chimneys and bay windows up to 2 feet. Decks under 30 inches above grade can project into rear setbacks. Variances require public hearing before the Planning Commission. ADUs have state-preempted reduced setbacks (4 ft side/rear) under Gov Code §65852.2. Coastal Zone projects may require Coastal Development Permits even for setback-compliant additions.