Costa Mesa, CA Environmental Rules: Flood Elevation Requirements (2026)

Section 13-82(d) of the zoning code lays out the construction standards Costa Mesa applies inside areas of special flood hazard. Beyond anchoring the structure against flotation and lateral movement and using flood-damage-resistant materials, subsection (d)(3) requires that new construction and any substantial improvement, defined elsewhere in the article as work costing 50% or more of the structure's appraised value, raise the lowest floor, including a basement, to or above the base flood elevation, which the code defines as the flood level with a 1% annual chance of being equaled or exceeded.

A registered professional engineer or surveyor must certify the finished elevation and submit that certification to the development services department. Nonresidential buildings get an alternative under subsection (d)(5): instead of elevating, they can be floodproofed watertight below the base flood level with components able to resist hydrostatic and hydrodynamic loads, again certified by a registered engineer or architect. Properties in Zone AO or Zone AH, where FEMA maps shallow, unpredictable flooding rather than a defined channel, follow subsection (d)(4) instead: the lowest floor must reach the depth number printed on the Flood Insurance Rate Map, or, if the map carries no depth number, at least two feet above the highest adjacent grade.

For enclosed areas below the lowest floor used only for parking, building access or storage, subsection (d)(6) requires flood-vent openings sized at one square inch of net opening per square foot of enclosed area, with the bottom of each opening no higher than one foot above grade, so floodwater can enter and exit without building up hydrostatic pressure on the walls.