O'Fallon, MO Environmental Rules: Flood Elevation Requirements (2026)

Non-residential buildings get an alternative: elevate the lowest floor two feet or more above base flood elevation just like a home, or flood-proof the structure below that level so it is watertight with walls able to resist hydrostatic and hydrodynamic forces, certified by a registered professional engineer or architect. Any fully enclosed area below the lowest floor used only for parking, building access or storage must be able to automatically equalize floodwater pressure, which the code satisfies with a minimum of two openings totaling at least one square inch of net area per square foot of enclosed space, with the lowest opening no higher than one foot above grade.

Manufactured homes placed in mapped flood zones must sit on a permanent foundation with the lowest floor two feet or more above base flood elevation and be anchored to resist flotation and lateral movement. Critical facilities, such as fire and police stations, hospitals and water or sewer treatment plants, are held to a tougher standard: they must be elevated above the 500-year flood level, or flood-proofed to that higher level, with access routes also above the 500-year flood elevation. The code's own definition ties freeboard directly to this cushion: an allowance against overtopping from waves, urbanization runoff or other factors the base flood calculation does not fully capture.