Sonoma County, CA Environmental Rules: Stormwater Management (2026)

Sonoma County's stormwater program is rooted in the federal Clean Water Act Section 402, California Water Code (Porter-Cologne Act), and the two MS4 permits issued by the North Coast and San Francisco Bay Regional Water Quality Control Boards. Chapter 11 of the Sonoma County Code (Construction Grading and Drainage) plus the County's Storm Water Management Program prohibit any non-stormwater discharge to County storm drains, gutters, or natural waterways. Allowed exemptions are limited to clean fire-suppression water, uncontaminated groundwater, and properly chlorinated swimming-pool drainage that meets discharge limits. Project applicants must implement five elements: public education, illicit-discharge detection and elimination, construction-site runoff controls (Erosion and Sediment Control Plans under GRD-011), post-construction runoff controls (Low-Impact Development - LID - best management practices), and good municipal housekeeping. LID requirements apply to most new development and significant redevelopment and may include bioretention, vegetated swales, pervious pavement, infiltration trenches, and rainwater capture sized to retain the 85th-percentile 24-hour storm. For projects in the Bay Area AQMD portion of the county, the C.3 provisions of the SFB MS4 permit apply (Regulated Projects creating or replacing 10,000 sq ft or more of impervious surface). Even construction projects exempt from the State Construction General Permit (under 1 acre disturbance) remain subject to Chapter 11's prohibition on polluted runoff and to BMP implementation year-round (with heightened requirements during the wet season).