5 county-level rules, plus city-specific rules for 4 cities in Washington County, Oregon.
Verified from official government sources
ADUs are allowed in unincorporated Washington County per Oregon state law (ORS 197.312(5)). Cities must allow ADUs on single-family lots. Oregon HB 2001 (2019) requires larger cities to allow middle housing. Parking minimums for ADUs are eliminated near transit under state law.
Accessory structures in unincorporated Washington County must meet setback requirements from property lines. Setback waivers are not permitted. Each land use district has unique setback requirements under CDC 418. Sheds under 200 sq ft are typically exempt from building permits under Oregon code.
Garage conversions to ADUs are permitted under Oregon state law requiring ADUs on residential lots. Building permits are required for all conversions. Construction must comply with Oregon Residential Specialty Code including fire separation requirements.
Washington County requires a building permit for carports. Carports must meet CDC 418 setbacks and CDC 413 parking/driveway rules. Detached accessory structures under 200 sq ft and 15 ft tall are exempt from building permits but not from zoning.
Washington County allows foundation-built tiny homes as dwellings or ADUs under CDC 430-89 (ADU) and ORS 197.312(5). Tiny homes on wheels are regulated as RVs or park model homes and cannot be occupied as permanent dwellings on residential lots.
4 cities in Washington County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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