Walla Walla County, WA Environmental Rules: Flood Zones (2026)

Walla Walla County Code Chapter 18.12, Flood Damage Prevention, adopted by Ordinance No. 447 in 2016, applies to every area of special flood hazard mapped in the county's January 2002 Flood Insurance Study and accompanying FIRM maps, on file with the Community Development Department. Section 18.12.120 requires a development permit before any construction or development, including fill, manufactured homes and other activities defined in the chapter, begins within that mapped area; the application must show the elevation of the lowest floor of proposed structures and, for floodproofed nonresidential buildings, an engineer's or architect's certification.

The Walla Walla County planning director administers the permit under Section 18.12.130. Section 18.12.190 requires all new construction and substantial improvements to be anchored against flotation, collapse or lateral movement, with detailed tie-down specifications for manufactured homes, including over-the-top and frame ties capable of carrying four thousand eight hundred pounds. Section 18.12.250 sets the elevation standard for new or substantially improved residential structures: the lowest floor, including any basement, must sit at or above one foot over the base flood elevation, and any enclosed area below that floor must either be prohibited or built with flood vents that automatically equalize hydrostatic pressure.

Construction inside a designated floodway is prohibited outright under Section 18.12.290 unless an engineer certifies no increase in flood levels, and within the rural residential Mill Creek 5 zoning district, Section 18.12.230(B) allows no building permit inside the mapped floodway or floodplain without a variance from the Hearing Examiner.