ADU rules in Benton County, WA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
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.
HB 1337 (2023), codified at RCW 36.70A.681, directs GMA-planning jurisdictions like Benton County to allow at least two accessory dwelling units on lots within an urban growth area that allow single-family homes — the county's UGAs are Kennewick, Richland, West Richland, Prosser, and Benton City. Inside those areas the county may not cap ADU size below 1,000 square feet, may not require owner-occupancy, and may not require off-street parking within half a mile of a major transit stop. Rural lots outside a UGA instead follow Benton County's rural zoning, which limits accessory dwellings more tightly. Any ADU still needs a building permit under the state building code.
Building or occupying an ADU without a Benton County building permit is enforced under RCW 19.27 with stop-work orders. County rules that conflict with RCW 36.70A.681 inside a UGA are unenforceable.
Other ordinances people look up for this city. Green dot = verified primary-source excerpt.
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See how Benton County's adu rules rules stack up against other locations.
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