Downey, CA Environmental Rules: Flood Elevation Requirements (2026)

Section 8710.06 sets Downey's construction standards for the special flood hazard areas established elsewhere in the chapter, and subsection (c) is the elevation and floodproofing rule. New residential construction must have its lowest floor, including any basement, elevated according to the zone: in an AR zone with a mapped base flood depth, the floor must sit above the highest adjacent grade at least as high as the depth number printed on the Flood Insurance Rate Map, or at least two feet if the FIRM lists no depth number at all. In an AR zone without a shown depth, the code instead uses whichever is lower: the AR base flood elevation, or three feet above the highest adjacent grade. Either way, once the structure is finished, a registered professional engineer or land surveyor has to certify the elevation of the lowest floor, including any basement, and file that certification with the Building Official before the project can close out.

Nonresidential buildings get an alternate path: they can either meet the same residential elevation standard, or floodproof instead, meaning the structure is built watertight below the base flood level, with walls substantially impermeable to water and components able to resist hydrostatic and hydrodynamic loads, certified by a registered engineer or architect. Section 8710.04 makes the Building Official responsible for keeping every one of these elevation and floodproofing certifications on file and available for public inspection, so a buyer or lender can confirm a structure actually meets the standard before relying on it.