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

Marin County Code of Ordinances § 23.09.034(c)(1) requires that new construction and substantial improvement of any structure have the lowest floor, including any basement, elevated to or above the base flood elevation (BFE), the water level expected during the flood that has a one percent annual chance of occurring (§ 23.09.030(4), (27)). The elevation of the completed lowest floor must be certified by a registered civil engineer or licensed land surveyor and filed with the floodplain administrator. In Zone AO or VO, § 23.09.034(c)(2) instead requires the lowest floor to 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 no depth number is shown.

Nonresidential buildings may substitute floodproofing under § 23.09.034(c)(3): a watertight structure below the BFE with components able to resist hydrostatic and hydrodynamic loads, certified by a registered engineer or architect. Before any of this is tested, § 23.09.033 requires the Department of Public Works to review every application for construction, grading, dredging, paving, drilling or manufactured-home placement inside a flood-prone area and to collect the proposed lowest-floor elevation. The public works director serves as floodplain administrator and must confirm a proposed project will not raise the base flood water surface more than one foot at any point (§ 23.09.033(d)(3)).

Where a watercourse will be altered, the administrator must notify neighboring communities and the California Department of Water Resources first (§ 23.09.033(d)(5)). Property owners inside a designated floodway face an added test under § 23.09.038: no fill, new construction or substantial improvement is allowed unless a registered civil engineer certifies it will not raise flood levels during the base flood discharge.