5 county-level rules, plus city-specific rules for 1 city in Clark County, Washington.
Verified from official government sources
In the urban growth area you may build up to two accessory dwelling units per legal lot, each capped at 1,000 sq ft (up to 1,500 sq ft on lots of 20,000+ sq ft). Rural zones (RADU) allow one, capped at 1,500 sq ft. No parking is required for urban
CCC 40.260.020(C)
Up to two (2) ADUs per legal lot are permitted and they must be accessory to a single-family detached dwelling... The total gross floor area of an ADU shall not exceed one thousand (1,000) square feet... On lots that are at least twenty thousand (20,000) square feet, the maximum square footage of an ADU may be increased up to one thousand five hundred (1,500) square feet.
Detached garden sheds, gazebos, and play houses may sit in side or rear setbacks in single-family zones. In residential (R1) zones, detached accessory buildings on lots 20,000 sq ft or smaller cannot exceed 18 feet in height; larger lots allow up to 35 feet. Sheds over 200 sq ft generally
CCC 40.260.010(E)
In the R1-5, R1-6, R1-7.5, R1-10 and R1-20 zoning districts, garden sheds, gazebos and play houses may be permitted in side or rear setbacks in the single-family residential zones subject to the provisions of Section 40.200.070(A)(4).
Clark County lets you convert an existing garage into an accessory dwelling unit, including a garage within nonconforming setbacks if the nonconformity predated April 26, 2024 and the conversion does not raise the garage's height. The conversion must meet building code and the ADU standards of CCC 40.260.020.
CCC 40.260.020(C)(1)(c)
Conversion of an existing garage, including existing garages within nonconforming setbacks; provided, that the nonconformity predated April 26, 2024, and the portion of the ADU within the setback does not increase the height of the existing garage.
Carports and detached garages are accessory buildings under CCC 40.260.010. In R1 residential zones they cannot exceed 18 feet in height on lots of 20,000 sq ft or less, or 35 feet on larger lots, and must meet the zone's setbacks. A freestanding garage can be built before the main
CCC 40.260.010(D)(1)
In areas zoned R1-20, R1-10, R1-7.5, R1-6 and R1-5: a. On lots less than or equal to twenty thousand (20,000) square feet in area, no accessory building shall exceed eighteen (18) feet in height. b. On lots larger than twenty thousand (20,000) square feet in area, no accessory building shall exceed thirty-five (35) feet in height.
Washington defines a tiny house as a dwelling that may be built on wheels and is no larger than 400 sq ft, excluding lofts. To be a legal permanent residence in Clark County a tiny home generally must sit on a foundation and comply with the Washington State Building Code
WA State Building Code / L&I Tiny Houses
A tiny house is a dwelling that may be built on wheels and is no larger than 400 square feet, excluding lofts.
1 cities in Clark County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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