Issaquah, WA Building Setbacks & Zoning: Lot Coverage Limits (2026)

Land Use Code § 18.404.040 is the city's lot coverage rule, written as an impervious surface standard rather than a building footprint cap. Its stated purpose is a balance of impervious and pervious surfaces on a lot, so that adequate drainage is achieved and runoff is controlled, with pervious surfaces providing scenic corridors and protection for water quality and the natural environment. The section warns that the standards assume no site constraints, so actual site constraints can keep an owner from using the full permitted allowance.

The total impervious surface of a site is the sum of the square footage of buildings and structures (their footprint), gravel and paved parking areas and driveways, gravel and paved walkways, sidewalks and bike paths, decks, patios, porches, tennis courts, sport courts and pools, and gravel and paved picnic and recreation areas. Buildings with vegetated roofs or minimal excavation foundations identified in the Low Impact Development Technical Guidance Manual for Puget Sound, dated December 2012, are excepted from the footprint count. Any other surface that prevents or retards water entering the soil counts too, but grass and sod do not.

Eaves and overhangs that extend 18 inches or less past the footprint are not counted. Overhangs beyond 18 inches, such as cantilevered decks and roof extensions, are counted, and the first 18 inches cannot be subtracted. Nothing is double counted: an overhang above an impervious terrace is skipped and the terrace is counted. Open, uncovered stormwater retention and detention facilities are not impervious, and neither is athletic artificial turf that acts as a stormwater facility, though artificial turf without subsurface stormwater facilities is.

The ratio divides the acres of impervious surface by the gross site area acreage. Gross site area includes any dedication for public right-of-way easement, and those dedications are not counted as impervious. The impervious and pervious ratios together equal 100 percent of gross site area. Required pervious areas must be landscaped under Chapter 18.606 IMC, and parking or pedestrian areas that use pervious pavers or pervious stormwater measures cannot be counted toward them. The areas that must be retained, in priority order, are critical areas that require buffers, existing significant trees under Chapter 18.812 IMC, and native vegetation areas. The percentage cap for each zone appears in the form and intensity tables at IMC 18.404.090 through 18.404.170, and Urban Village zones reach up to 100 percent.