Can a basement become an ADU in Portland?
Yes. As of October 2026, converting part of the existing house is one of the ways Portland's zoning code allows an ADU to be created, and a basement is the most common example.
Does the size limit apply to a basement ADU?
Not always. Portland's 800 sq ft / 75% cap does not apply to an ADU in the basement of a house that is at least five years old. In an older house, the entire basement can become the unit if the building code allows it. Read the size guide.
What does the building code look at?
Zoning says the unit is allowed; the building code decides whether the space is livable. The usual checks:
- Ceiling height. Low ceilings, beams and ducts may need to be raised, rerouted or the floor lowered.
- Egress. Sleeping rooms need an emergency escape opening, which often means a larger window and a window well.
- Moisture. Drainage, a sump if needed, and insulation detailing that keeps the walls dry.
- Fire separation. A rated ceiling and protected stair between the two units.
- Entry. A separate exterior door, often by an exterior stair.
When is a basement hard to convert?
It depends on what is down there. A basement with headroom under 7 feet, a furnace and water heater in the middle of the floor, or seasonal water on the floor can need major work before it becomes a home. We check ceiling height, drainage and mechanical layout on the first visit.
Can I also build a backyard ADU?
Often. On a qualifying Portland lot, a basement unit can pair with a detached ADU, since only one of the two may be attached to the house. See the two-ADU 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 basement
Send your address. We check the size rules and look at headroom, egress and drainage on a visit.
Serving Portland, Beaverton, Lake Oswego, Milwaukie and Gresham.