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

King County regulates lot coverage through two related measures. Building lot coverage is the percentage of a lot covered by buildings and structures, regulated under KCC 21A.12. Impervious surface coverage includes all hard surfaces (buildings, driveways, patios, walkways) that prevent water infiltration, regulated under KCC 21A.24 Critical Areas and the King County Surface Water Design Manual. Urban residential zones vary: R-1 typically allows 35 percent building coverage, R-4 allows 45 percent, R-6 allows 50 percent, and R-8 allows 55 percent. Rural Area zones (RA-2.5, RA-5, RA-10) limit impervious surface to 8 percent of the lot or 15,000 square feet, whichever is less, to protect groundwater recharge and prevent stormwater runoff that degrades salmon streams. Agricultural zones have even tighter limits. Parcels within critical aquifer recharge areas, near salmon-bearing streams, or on steep slopes have additional impervious coverage restrictions. Any new or replaced impervious surface over 2,000 square feet triggers the King County Surface Water Design Manual stormwater requirements. Landscaped areas, pervious paving, and rain gardens do not count against impervious coverage limits. Variances require hearing before the Hearing Examiner with demonstrated hardship.