San Mateo County, CA Fence Regulations: Retaining Walls (2026)

San Mateo County does not exempt retaining-wall fences the way it exempts simple wood and cyclone fences. The county's fence guidance states that a building permit is required when a fence is constructed above a retaining wall. The retaining wall itself is regulated under the California Building Code, which the county adopts. CBC Section 105.2 exempts retaining walls that are not over 4 feet in height measured from the bottom of the footing to the top of the wall, unless the wall supports a surcharge (such as a slope, driveway, or structure above it) or impounds certain liquids — in which case a building permit is required regardless of height. For fence height purposes, San Mateo County measures fences from the lowest adjacent natural grade, so a fence sitting atop a retaining wall can quickly exceed the 4-foot front or 6-foot side/rear limits and may require a Fence Height Exception in addition to the building permit. Section 8.332.020 of the Zoning Regulations addresses abrupt grade shifts at a boundary: where there is an abrupt change in land height between two owners, the lower owner may build a fence, wall, or hedge to the otherwise-applicable height limit but in no event more than 12 feet high. Engineered plans and drainage details are typically required for permitted walls.