5 county-level rules, plus city-specific rules for 1 city in Yakima County, Washington.
Verified from official government sources
Washington's HB 1337 (RCW 36.70A.681) requires Yakima County to allow at least two accessory dwelling units on lots within its urban growth areas, cannot cap ADUs below 1,000 square feet, and cannot require owner-occupancy.
RCW 36.70A.681(1)(b)
The city or county may not require the owner of a lot on which there is an accessory dwelling unit to reside in or occupy the accessory dwelling unit or another housing unit on the same lot.
A one-story detached shed of 200 square feet or less needs no building permit in unincorporated Yakima County under the state-adopted IRC (WAC 51-51), but it must still meet county setbacks and cannot have plumbing, wiring, or heating.
IRC 2018, Section R105.2 (WAC 51-51)
One-story detached accessory structures used as tool and storage sheds, playhouses and similar uses, provided the floor area is not greater than 200 square feet (18.58 m2).
Converting a garage into living space in unincorporated Yakima County is a change of occupancy needing a building permit under the state building code, and if it adds a kitchen and separate entrance it becomes an ADU under HB 1337.
A carport is an accessory structure in unincorporated Yakima County and needs a building permit under the state building code, with engineered anchorage for Yakima Valley wind and snow loads, plus compliance with county accessory-structure setbacks.
A tiny home on a permanent foundation is a dwelling in unincorporated Yakima County and may use IRC Appendix Q, while a tiny home on wheels is an RV that cannot be a permanent residence in a residential zone.
IRC 2018, Appendix Q, Section AQ102 (WAC 51-51)
A dwelling that is 400 square feet (37 m2) or less in floor area excluding lofts.
1 cities in Yakima County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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