Stormwater

Stormwater Management in Development Projects

The State's Water Board has adopted stringent stormwater management requirements for development projects that replace or add impervious surfaces (surfaces that water cannot pass through).

C.3 Regulated Projects- New Changes in Effect: The following projects are currently regulated under Section C.3 of the Municipal Regional Stormwater NPDES Permit (MRP) and need to actively manage stormwater treatment on-site through source control measures. These projects require completion of the C.3 and C.6 Development Review Checklist (download below) with the building permit application:

• Stormwater treatment requirements will apply to most projects that create and/or replace 5,000 square feet or more of impervious surface.
• Public right-of-way areas in front of development projects that are as part of the project must now be included in impervious surface calculations and runoff must be treated from those areas.
• Large single-family homes creating and/or replacing 10,000 square feet or more of impervious surface will now have treatment requirements.

Small Projects: The following projects are currently exempt from regulation under Section C.3 of the MRP, but still need to incorporate at least one site design measure to reduce stormwater runoff into their plans. These projects require completion of the Small Projects Checklist (download below) with the building permit application:

• Standalone single-family home (includes ADUs) creating and/or replacing less than 10,000 square feet of impervious surface (note: only applies to development of one single-family dwelling and/or ADU on one site).
• Restaurants, retail gasoline outlets, auto service facilities  and parking lots (stand-alone or part of another use) that create and/or replace less than 5,000 square feet of impervious surface
• Any other type of project that creates and/or replaces less than 5,000 square feet of impervious surface.

Note that the C.3/C.6 and Small Projects checklists are an initial screening form/worksheet and depending on your project, supplemental materials may also be required.

For further information regarding construction and stormwater, please visit the San Mateo County - Flows to Bay website.  For detailed technical information on requirements for C.3 regulated projects, including the C.3 Technical Guidance handbook, go to Flows to Bay Technical Guidance. For the C.3 regulations, refer to the California Regional Water Quality Control Board Municipal Regional Stormwater NPDES Permit.