Ada County, ID Environmental Rules: Flood Zones (2026)

and applies to "development" within all Special Flood Hazard Areas shown on the Flood Insurance Study for Ada County (revised June 19, 2020) and its accompanying FIRM and DFIRM maps, plus foothill or hillside tributary floodplains draining more than ten acres. The Floodplain Administrator, a role assigned within Development Services, reviews and permits all development in these areas, confirms compliance with Section 404 of the Clean Water Act, and coordinates any watercourse relocation with the Idaho Department of Water Resources before it happens. Inside the regulatory floodway and the AE, A and D flood zones, the code flatly bars new construction or substantial improvement of critical facilities along with uses such as explosive manufacturing, flammable substance storage, gasoline sales, junkyards, group foster homes, sanitary landfills and manufactured home storage, regardless of any other zoning designation.

Development along the Boise River floodway must also clear a 100 foot structure, parking and fence setback from the floodway line, plus an added 5 foot buffer from any county recreational easement wider than 65 feet, and grading or vegetation clearing is barred within 100 feet of the FEMA regulatory floodway boundary. Where Great Blue Heron rookeries or Bald Eagle nests are documented along the Boise River Greenway, setbacks of 300 feet and 200 feet apply respectively. Floodplain permits run for two years, and appeals of Floodplain Administrator decisions go to the Board of County Commissioners under Section 8-6-13, supported by a professional engineer's certification of elevations.