Issaquah, WA Environmental Rules: Flood Elevation Requirements (2026)

The measuring stick in Issaquah is the flood protection elevation, defined in IMC 16.36.030 by the area of special flood hazard designation on the FEMA flood insurance rate map (FIRM) and the Issaquah Creek Basin and Nonpoint Action Plan. For residential and inhabited structures, including manufactured homes, and the equipment serving them, the level is 2 feet above base flood elevation in Zone AE, 2 feet in Zone AH, and 3 feet in Zone A. In Zone AO it is 2 feet above the depth number on the FIRM or 3 feet above the highest ground surface, whichever is greater. For all other activities or uses designated by the Floodplain Administrator, such as uninhabited structures, parking and non-temporary outdoor storage, the level is 1 foot above base flood elevation in Zones AE, AH and A, and in Zone AO 1 foot above the FIRM depth number or 2 feet above the highest ground surface, whichever is greater. Key facilities use the higher figures: 2 feet in Zones AE and AH and 3 feet in Zone A.

Section 16.36.130(A) requires new residential construction, substantial improvement, and improvement of any post-FIRM residential structure to have the lowest floor, including the basement, and all electrical, heating, ventilation, plumbing and air-conditioning equipment elevated at or above that level. Fully enclosed areas below the lowest floor that are subject to flooding are prohibited, or if used solely for parking, access or storage must have a minimum of 2 openings with a total net free area of not less than 1 square inch for every square foot of enclosed area, with the bottom of all openings no higher than 1 foot above the adjacent exterior grade.

Nonresidential buildings under 16.36.130(B) must meet the same elevation, or ASCE 24 if that is greater, or be dry floodproofed above the base flood level with an engineer's certification. Applicants floodproofing a nonresidential building are told that insurance premiums will be based on rates 1 foot below the floodproofed level. Critical facilities inside the floodplain need the lowest floor elevated to 3 or more feet above base flood elevation. Under 16.36.100(B) an approved elevation certificate is required before final inspection.