What is an attached ADU?
It is a complete second unit joined to the main house, usually by a shared wall. It can be an addition to the rear or side, a unit built over an attached garage, or a part of the existing house closed off as its own home.
How large can an attached ADU be?
As of October 2026, Portland caps it at 800 sq ft or 75% of the main house's living area, whichever is smaller. The house's base zone standards, such as setbacks and height, still apply to the combined building.
Can I have an attached ADU and a detached one?
On many lots, yes. Portland allows two ADUs with a house when the site qualifies, but only one may be attached to the house or to another ADU. Pairing an attached unit with a backyard cottage is allowed; two attached units are not. See the two-ADU guide.
What makes an addition more complex?
- Tie-in. The new roof and walls must join the old house cleanly and keep water out.
- Fire separation. A rated wall and ceiling between the two units.
- Sound. Insulation and detailing so the households do not hear each other.
- Utilities. Panel capacity and plumbing routes from the existing house.
Addition or detached cottage?
It depends on the house. An addition can reuse a foundation wall and shorten utility runs, but tying into an old roof or foundation can add surprises. A cottage costs more in structure but keeps the work away from your living space. We price both if your lot allows them.
What does an attached ADU cost?
A 2026 national guide from Block Renovation put attached ADUs at about $100,000 to $250,000, as of October 2026. Portland prices vary. See the Portland cost guide.
Related
Rules as of October 2026, from the City of Portland ADU zoning summary (Zoning Code Chapter 33.205) and ORS 197A.425. Suburban cities differ; see areas.
Check your lot
Send your address. We look at side and rear setbacks and where an addition could go.
Serving Portland, Beaverton, Lake Oswego, Milwaukie and Gresham.