Water resources are crucial to San Mateo County. Water provides recreation to San Mateo County residents and visitors. It is also home to many types of wildlife, including endangered species like salmon, steelhead trout, and California red-legged frog. Water pollution degrades the quality of surface waters, making them unsafe for drinking, fishing, swimming, and other activities. Excessive surface water runoff from private properties could flood the public right of way and adjacent properties, impacting intactness of natural resources, public safety, and property.

Water resource protection and flood prevention are everyone's responsibility. For all the projects in the County’s unincorporated area that require building and planning permits, the San Mateo County Planning and Building Department enforces the following stormwater-related policies:

1. For all projects requiring a demolition, grading, or building permit that involves site disturbance, the submittal of an Erosion and Sediment Control Plan for County review and approval is required. Also see the Erosion and Sediment Control Plan requirements.
2. For all projects requiring a planning or building permit that would result in any new/replaced area(s) of impervious surface, the following policies apply:
   1. San Mateo County Planning and Building Department, together with, the San Mateo Water Pollution Prevention Program (SMCWPPP), enforces the State requirements for stormwater quality control and provides guidance resources.
   2. San Mateo County Planning and Building Department enforces the surface water runoff flow control, or drainage, on private properties with detailed requirements and applicable forms.
3. For coastal projects, ASBS requirements are enforced. The State Water Resources Control Board adopted the California Ocean Plan on July 6, 1972 with the latest revisions adopted in 2009. The Ocean Plan prohibits the discharge of waste to designated Areas of Special Biological Significance (ASBS). ASBS are designated by the State Water Board as ocean areas requiring protection of species or biological communities to the extent that alteration of natural water quality is undesirable.
4. For all projects disturbing the public right of way, San Mateo County Department of Public Works enforces Encroachment Permit requirement.