Multnomah County, OR Environmental Rules: Flood Elevation Requirements (2026)

Have the lowest floor, including basement, elevated to at least one foot above the base flood elevation. All manufactured dwellings to be placed or substantially improved shall be elevated on a permanent foundation such that the finished floor of the manufactured dwelling is elevated to a minimum of 18 inches above the base flood elevation. The bottom of the lowest chassis frame beam for all manufactured dwellings subject to this provision shall be at least 12 inches above the base flood elevation. Floating dwellings do not need to be elevated but must be able to rise with flood waters to the design flood elevation required by this subsection. This will require consideration of the piling heights. The lowest floor, including basement, shall be elevated to at least two feet above the highest adjacent grade where flood elevation data is not available either through the Flood Insurance Study, FIRM, or from another authoritative federal, state or other source. Where flood elevation data is not available, a State of Oregon registered professional engineer or architect shall also verify that the proposed construction will be reasonably safe from flooding.