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

Washington County's Community Development Code regulates construction inside floodplains and drainage hazard areas mapped on the Flood Insurance Rate Map effective October 19, 2018. 1 requires the lot to contain buildable land outside the flood area sufficient to permit construction at least 1 foot above the delineated flood surface elevation, with a minimum 10-foot perimeter setback around the dwelling that stays outside the flood area. 2 sets construction standards: the lowest floor, including any basement, must be elevated to at least 1 foot above the flood surface elevation and anchored to resist flotation, collapse or lateral movement.

Builders may instead use pressure-treated pilings if a registered engineer certifies they will withstand a 100-year flood, the pilings sit on stable compacted fill no more than 10 feet apart, and any piling over 6 feet high is reinforced. Approved fill is also allowed if the building site, plus a 10-foot perimeter, sits above the flood surface elevation. All construction materials and utility equipment must resist flood damage. Any fully enclosed area below the lowest floor that can flood must automatically equalize hydrostatic pressure: at least two openings totaling 1 square inch of net area per square foot of enclosed space, with the bottom of each opening no higher than 1 foot above grade.

Non-dwelling structures face a parallel standard under Section 421-10. These rules apply only in the unincorporated county; Hillsboro, Beaverton and the other cities inside the county administer their own floodplain codes.