San Mateo County, CA Environmental Rules: Grading & Drainage (2026)

Grading permits are governed by the County's grading ordinance (Title 6) in conjunction with the California Building Code Chapter 18 (Soils and Foundations) and Appendix J (Grading). Threshold for permit: more than 250 cubic yards of cumulative cut and/or fill, OR any grading on slopes 20% or steeper, OR grading within 100 feet of a watercourse, OR cut/fill over 3 feet in height, OR any grading in the Coastal Zone. Grading permit applications require: topographic survey, grading plan showing existing and proposed contours, drainage plan, erosion and sediment control plan, geotechnical report (for sites on fill, steep slopes, or with liquefaction potential), hydrology analysis (if within 100 ft of watercourse), and tree protection/removal plan. Drainage from the site cannot be redirected onto a neighboring parcel in a way that causes damage (common-law nuisance plus Civil Code §§1008, 3479). Retaining walls over 4 feet in height or supporting a surcharge (driveway, slope) require structural engineering stamped by a California PE and a building permit. Large grading projects may need to implement the Construction General Permit (1+ acre disturbance) with a SWPPP. Compaction testing required for structural fill (typically 90-95% relative compaction).