What Is an Impervious Surface?

An impervious surface is a surface covering or pavement of a developed parcel of land that prevents the land’s natural ability to absorb and infiltrate rainfall. Impervious surfaces include, but are not limited to: rooftops, walkways, patios, driveways, parking lots, storage areas, impervious concrete and asphalt, and any other continuous watertight pavement or covering.

Does Pervious Paving Count as Impervious Surface?

Pervious paving, including pavers with permeable openings and seams, is not considered impervious if it is underlain with pervious soil or pervious storage material, such as a gravel layer that is sized to hold the volume of stormwater runoff specified in Provision C.3.d of the MRP (80 percent of the average annual runoff). Guidance for calculating this amount of runoff is provided in Section 5.1 of the San Mateo Countywide Water Pollution Prevention Program’s (Countywide Program) C.3 Technical Guidance.

Is my project a “Regulated” project?

In general, Regulated Projects are:

  • Create and/or replace greater than 10,000 sq ft of impervious surface (5,000 sq ft for restaurants, gas stations, auto service facilities, and/or uncovered parking area)
  • Not stand-alone single family homes (though single-family homes within a subdivision could be subject to regulation)

See the C.3 and C.6 Development Review Checklist and C.3 Regulated Projects Guide for more information. Regulated projects are subject to stormwater treatment requirements per provision C.3 of the County’s Municipal Regional Permit (MRP) with the Regional Storm Water Control Board. 

My project is NOT a “Regulated” project – why is it still subject to drainage requirements?

San Mateo County Planning & Building reviews projects to confirm stormwater is managed in such a way that it will not negatively impact structures onsite, neighboring properties, and/or the community infrastructure. To accomplish this, SMC has developed guidance to restrict stormwater flows across property lines, with varying levels of review and requirements depending on the project characteristics, including amount of impervious surface proposed. These requirements generally apply to projects greater than 150 sq ft of new and replaced impervious surface – see the San Mateo County Drainage Manual for more information.