Redwood City, CA Environmental Rules: Flood Elevation Requirements (2026)

18's elevation and floodproofing standards apply throughout areas of special flood hazard. For residential new construction or substantial improvement, the lowest floor, including any basement, must be elevated to a minimum of one foot above the base flood elevation or the design flood elevation, whichever is higher, in an A zone, and the same one-foot minimum applies in all other zones. Nonresidential new construction or substantial improvement may either meet that same elevation standard or be dry floodproofed instead, so the structure is watertight with walls substantially impermeable to water, has structural components able to resist hydrostatic and hydrodynamic loads, and is certified by a registered engineer or architect to the Floodplain Administrator.

Enclosed areas below the lowest floor used only for parking, building access or storage must be designed to automatically equalize floodwater pressure, generally through at least two openings totaling one square inch of net area per square foot of enclosed area, with the bottom of openings no higher than one foot above grade, or through an engineer's or architect's certification. 6 defines them as "breakaway walls" that must be designed to break away under abnormally high tides or wave action without damaging the structure's integrity, must have a safe design-loading resistance of not less than ten and no more than twenty pounds per square foot, and must be certified by a registered engineer or architect. 18(C)(3).