Benton County, OR Accessory Structures: ADU Rules (2026)

Only one ADU is allowed per single-family dwelling, and the primary house must exist before or at the same time the ADU is built. The ADU counts as interior, attached, or detached space, but a single-car garage up to 300 square feet may be added without counting toward the 900-square-foot habitable cap, and that garage cannot be used as living space. Water and sewer service must be secured before permitting: the ADU can tie into an existing or new septic system approved by Benton County Environmental Health and the Oregon DEQ, or connect to a municipal sewer system with written evidence the provider can serve it.

050(6) raises the required flow-test margin and sets modified minimum supply and storage standards, down to specific gallon-per-minute storage tables. The ADU must share the same road approach as the main dwelling, sit no more than 200 feet away measured wall to wall, and meet any applicable Fire District access requirements. A manufactured home can serve as the ADU if it otherwise meets the standards. Because most of Benton County's unincorporated land sits in urban fringe areas destined for eventual annexation, applicants must also submit an urbanization plan showing the site can still reach the respective city's minimum density, and that plan binds future owners until it is amended or the property is annexed. 050(15) requires a recorded covenant memorializing the size, bedroom/bathroom, and urbanization-plan conditions so they run with the land.