San Mateo County, CA Environmental Rules: Flood Elevation Requirements (2026)

300 defines the base flood as the flood having a one percent chance of being equaled or exceeded in any given year, sometimes called the 100-year flood; that base flood is the basis for the regulatory flood-protection elevation used throughout the chapter. The lowest floor is the lowest enclosed area of a structure, including basement areas, though it excludes an unenclosed area or flood-resistant enclosure used solely for vehicle parking and building access or storage areas that meet the construction standards in Sections 8131 through 8133 of the Building Regulations.

060(1)(a), a development permit for an Area of Special Flood Hazard issues only when the structure is built so its lowest floor is elevated to or above the regulatory flood-protection elevation, and the project also meets the Standards of Construction in Section 8131, the Manufactured Homes standards in Section 8132 and the Water Supply and Sewage Systems standards in Section 8309. The Director of Public Works must obtain, review and use any available federal, state or other base flood elevation and floodway data, applying it to require that new residential construction have its lowest floor elevated to or above the base flood level, while non-residential structures may instead be floodproofed to that level.

Elevation certifications for each project, plus manufactured-home and floodproofing certifications, are kept on file in the Building Inspection Section for public inspection and for National Flood Insurance Program purposes. A separate cumulative-impact standard caps the effect of new development: combined with all other existing and anticipated development, a project cannot raise the base flood's water surface elevation by more than one foot at any point before a permit can issue.