What is a detached ADU?
It is a separate small house on the same lot as your home, sometimes called a backyard cottage. It offers the most privacy of any ADU type, which matters to tenants and to family members who want their own front door.
Where can it go on my lot?
As of October 2026, Portland requires a detached ADU to sit at least 40 feet back from the front lot line or behind the rear wall of the house. Its building coverage cannot exceed the house's, and all detached accessory structures together may cover no more than 20% of the site.
What height limit applies?
Up to 20 feet where it stands outside the required setbacks, and up to 15 feet inside them. That 20-foot allowance can fit a loft or a compact second floor on many lots.
Are there design rules?
Yes, once the unit is taller than 15 feet. Portland then asks the roof pitch, trim, eaves, windows and siding to either match the house or meet set minimums, such as a roof pitch of at least 6/12 and eaves of at least one foot. Lots in historic areas follow separate standards or a Historic Resource review.
How big can it be?
As of October 2026, 800 sq ft or 75% of the house's living area, whichever is smaller. A 1,000 sq ft house therefore caps the unit at 750 sq ft. Read the size guide for how living area is measured.
When is a detached ADU the wrong choice?
It depends on what the backyard holds. A big tree near the build site, a steep slope, a long or uphill sewer run, or tight access for equipment can all push the cost up. If your house has a usable basement, that unit may cost far less. We compare options on the site visit.
What does a detached ADU cost?
A June 2026 national cost guide from Block Renovation put detached ADUs at about $150,000 to $400,000 all in. Portland numbers can fall outside that range. 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 backyard
Send your address. We check setbacks, coverage and the sewer route, then sketch what fits.
Serving Portland, Beaverton, Lake Oswego, Milwaukie and Gresham.