Bay Area 5,000-Square-Foot Stormwater Rule for Pavement Projects

The San Francisco Bay Regional Water Quality Control Board's Municipal Regional Stormwater Permit, commonly called the MRP, requires covered municipalities to implement post-construction stormwater controls. Beginning July 1, 2023, specified project categories use a 5,000-square-foot impervious-surface threshold. The permit includes detailed categories and transition provisions, so the number alone is not a complete applicability test.

A regulated project may need Low Impact Development site design, source controls, stormwater treatment, hydraulic sizing, operation and maintenance commitments, and municipal documentation. A local reviewer applies the permit through city or county ordinances, technical guidance, application forms, and approval conditions.

The MRP applies to named municipal permittees in the San Francisco Bay region. Other California areas operate under different Regional Water Board permits, the statewide Small MS4 permit, or other municipal programs. Some also use 5,000 square feet for particular categories, but the definitions and requirements are not interchangeable.

Identify the property address, governing city or county, receiving stormwater program, and current permit before using any threshold. For the broader state framework, read California Pavement Regulations and Requirements.

Measure existing impervious surface to remain, existing surface to be removed, replaced impervious surface, new impervious surface, and pervious area separately. Conventional asphalt, concrete, roofs, walkways, driveways, and parking areas are common impervious surfaces. The permit and local guidance control how contiguous areas, project boundaries, and permeable systems are counted.

Sealcoating, crack treatment, isolated patching, an overlay, cold planing, and full-depth reconstruction change the site in different ways. A coating may leave the underlying pavement in place, while reconstruction may remove and replace the pavement section and base. Local definitions and the full work description determine whether a scope is maintenance, redevelopment, road reconstruction, or another category.

Do not rely on a contractor's label alone. Provide demolition depths, plan limits, drainage changes, curb work, widening, and connected improvements to the municipal reviewer. Keep the agency response with the project file.

Start early enough to change the layout. Stormwater treatment can require space, elevations, overflow paths, utility coordination, landscaping, access, and a recorded maintenance obligation. A late determination can affect bids and construction timing.

Low Impact Development manages runoff near its source. Depending on the project and local criteria, a design may use bioretention, flow-through planters, permeable pavement, rainwater capture, or another approved measure. The permit does not make one treatment type suitable for every site.

See Permeable Pavement for California Properties and Bioretention for California Pavement Projects for feasibility and owner-review questions.

Municipal post-construction review focuses on the completed project's runoff and permanent controls. California's Construction Stormwater General Permit focuses on pollutants and discharges during construction and generally applies when one acre or more of soil is disturbed, or when a smaller site is part of a larger common plan that reaches one acre. A project can trigger one program, both, or neither while still needing local erosion controls.

For construction coverage, qualified roles, SMARTS, and closeout, use California SWPPP Planning for Paving Projects.

Give the reviewer a site plan and complete scope, then retain the answer with the permit record.

This guide was reviewed against current primary sources on August 2, 2026. Confirm the current permit and local guidance before using any threshold.