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

Section 20.255.110 sets the construction standards that apply throughout San Marcos's special flood hazard areas, and subsection C (Elevation and Floodproofing) fixes the City's freeboard requirement above FEMA's base flood elevation. For residential construction, the lowest floor, including any basement, must sit at least two feet above the base flood elevation in Zones AE, AH, and A1-30, and in an unnumbered Zone A without a published base flood elevation the same two-foot freeboard applies once the Floodplain Administrator determines the elevation under Section 20.255.090.C. In Zone AO, the lowest floor must clear the highest adjacent grade by two feet above the FIRM's published depth number, or by at least four feet if no depth number is shown.

Non-residential buildings may meet the same elevation standard or instead be dry-floodproofed watertight below that elevation, with structural components able to resist hydrostatic and hydrodynamic loads, certified by a registered civil engineer or architect under Section 20.255.110.C.2. Enclosed areas below the lowest floor used only for parking, access, or storage must have flood openings sized to at least one square inch per square foot of enclosed area, with the lowest opening no higher than one foot above grade, so hydrostatic pressure equalizes automatically during a flood.

Manufactured homes face the same two-foot freeboard when placed outside a mobile home park, per Section 20.255.140. Once construction is finished, a registered civil engineer or licensed land surveyor must certify the as-built lowest-floor elevation, and the community building inspector verifies that certification before the City signs off, with the paperwork kept on file for public inspection under Section 20.255.090.E.