Washington County, OR Building Setbacks & Zoning: Lot Coverage Limits (2026)

Lot coverage in Washington County is regulated by CDC Chapter III zone tables. Typical caps: R-5 — 50% coverage; R-6 and R-9 — 50-55%; R-15 — 60%; R-24 and R-25+ — 60-70%. AF-5, AF-10, AF-20, and EFU — structures cannot exceed a fixed percentage (often 25%) or specific square-footage caps. Coverage counts all buildings, accessory structures, covered porches, and decks over 30 inches; open decks, driveways, and walkways may count toward Maximum Impervious Surface separately. Clean Water Services (CWS), the county's regional wastewater and stormwater utility, enforces post-construction stormwater standards under Design & Construction Standards §4: any project creating 1,000 sq ft or more of new/replaced impervious surface requires on-site stormwater management (rain garden, infiltration, detention). This triggers on most additions, new homes, and patios in unincorporated areas. Permeable pavers can receive 50% credit toward the impervious calculation if engineered. Cities (Beaverton, Hillsboro, Tigard, Tualatin, Sherwood, Cornelius, Forest Grove, North Plains, Banks, Gaston) each set their own lot coverage standards often aligned with county numbers. HB 2001 Middle Housing code updates have loosened lot coverage in many residential zones.