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

1) applies inside the urban growth boundary in unincorporated Washington County, letting a homeowner add up to two accessory units to a single-family lot through a Type I development permit, decided by county staff without a public hearing. Only one ADU may be attached to or built inside the primary house if two units are proposed; the other must be detached or carved from a detached accessory building such as a garage. An attached unit is capped at 800 square feet of floor area, but the code carves out an exception: it may take up an entire floor of the primary dwelling or an existing attached garage even past 800 square feet.

A detached unit is also capped at 800 square feet, rising to 920 square feet if built to Uniform Federal Accessibility Standards. A detached ADU must sit at or behind at least one street-facing wall of the house, keep a 5-foot side yard, and match the rear-yard setback of the underlying zoning district. Height cannot exceed the primary dwelling's height. The ADU may be occupied before the primary house is finished, but whichever unit is occupied first counts as the primary dwelling for System Development Charge and Transportation Development Tax purposes.

Property outside the urban growth boundary follows the separate Rural ADU standard at CDC § 430-3, requiring a lot of at least two acres, capping the unit at 900 square feet, and requiring it sit within 100 feet of the primary house. Beaverton, Hillsboro, Tigard and the county's other cities administer their own ADU codes; this standard covers only unincorporated land.