5 county-level rules, plus city-specific rules for 1 city in Benton County, Washington.
Verified from official government sources
Washington's HB 1337 (RCW 36.70A.681) requires Benton County to allow at least two accessory dwelling units on single-family lots inside its urban growth areas, cannot cap ADUs below 1,000 square feet, and cannot require owner-occupancy.
RCW 36.70A.681(1)(c)
The city or county must allow at least two accessory dwelling units on all lots that are located in all zoning districts within an urban growth area that allow for single-family homes.
A one-story detached shed of 200 square feet or less needs no building permit in unincorporated Benton County under the 2021 state building code (WAC 51-51), but it must still meet county setbacks and stay unheated and uninhabited.
Converting a garage into living space in unincorporated Benton County is a change of occupancy needing a building permit, and if it gains a kitchen and separate entrance it becomes an ADU under HB 1337.
A carport is an accessory structure in unincorporated Benton County and needs a building permit, with posts and roof anchored against Columbia Basin winds, plus compliance with county accessory-structure setbacks.
A tiny home on a permanent foundation is a dwelling in unincorporated Benton County and may use IRC Appendix AQ, while a tiny home on wheels is an RV that cannot be a permanent residence in a residential zone.
1 cities in Benton County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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