Unincorporated San Mateo County requires building permits for retaining walls over 4 feet measured from bottom of footing to top of wall, per California Building Code. Engineered plans required. Steep terrain common on the Peninsula often triggers geotechnical review.
San Mateo County Building Regulations adopt the California Building Code. Retaining walls over 4 feet measured from the bottom of the footing to the top require a building permit (CBC §105.2 exception). Walls supporting surcharge loads (driveways, structures, slopes) require a permit at any height. Permits require engineered plans stamped by a California-licensed civil or structural engineer. Geotechnical soils reports commonly required on the Peninsula's steep terrain (Emerald Hills, West Menlo Park, Skyline, Coastside bluffs) due to seismic (San Andreas Fault runs through county) and landslide hazards. Drainage behind walls must be addressed with perforated pipe and gravel backfill. Setback requirements apply for walls near property lines (typically 1-3 feet plus drainage). Coastal zone walls and walls over 6 feet in scenic corridors may require discretionary Planning review. Shotcrete, keystone, and gravity walls all regulated.
Unpermitted walls: stop-work order, retroactive permit with 2x fee, possible demolition if non-engineered. Fines $500-$2,500. Walls causing property damage: civil liability and potential slope stability remediation costs.
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See how San Mateo County's retaining walls rules stack up against other locations.
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