Mendocino County, CA Fence Regulations: Retaining Walls (2026)

Retaining walls in unincorporated Mendocino County are regulated primarily through Title 18 (Building Regulations), which adopts the California Building Code. Consistent with the statewide exemption, a retaining wall not over four (4) feet in height, measured from the bottom of the footing to the top of the wall, does not require a building permit unless it supports a surcharge (such as a slope, driveway, or structure loading the wall) or impounds flammable liquids. Walls taller than four feet, or any wall supporting a surcharge regardless of height, require a building permit and engineered design through the Building Division. Because much of the County is hilly and geologically active, retaining walls are often tied to grading work, which is separately regulated by the Excavation and Grading ordinance in Chapter 18.70; that chapter requires a grading permit for regulated excavation and fill, with thresholds and exemptions set out in Section 18.70.030. In the Coastal Zone, retaining walls and the grading they require can constitute 'development' subject to the Division II Coastal Zoning Code, potentially requiring a Coastal Development Permit, and blufftop or shoreline walls face additional geologic setback and erosion-protection review. Property owners should confirm with Planning and Building Services whether a building permit, grading permit, and/or coastal permit applies before constructing a retaining wall.