As of October 2026, a Portland ADU can be up to 800 sq ft or 75% of the main house's living area, whichever is smaller. A basement ADU in a house at least five years old is exempt from that cap. A detached ADU can be 20 feet tall outside required setbacks and 15 feet inside them.
How is living area measured?
Portland's living area excludes exterior wall thickness, garages, basement areas with ceilings under 6 feet 8 inches, and spaces under 5 feet tall or not reached by a stair. That matters twice: once for the ADU and once for the house it is compared with.
Can you show the math?
- A house with 1,400 sq ft of living area: 75% is 1,050, so the 800 sq ft cap controls.
- A house with 900 sq ft of living area: 75% is 675 sq ft, which becomes the cap.
- A basement unit in a 1920s house: the size cap does not apply.
What other limits shape a detached ADU?
- At least 40 feet from the front lot line, or behind the rear wall of the house.
- Building coverage no larger than the house's.
- All detached accessory structures together at no more than 20% of the site.
- Design standards on roof, trim, eaves, windows and siding once it is taller than 15 feet.
Do the suburbs use the same limit?
No. Gresham allows 900 sq ft or 75% of the house's floor area, whichever is less. Lake Oswego allows 800 sq ft of gross floor area, with an exception for some conversions. See the area pages.
Can I go bigger than the limit?
It depends on the standard. Some limits can be adjusted through Portland's Adjustment Review, which the city describes as discretionary, about 8 to 10 weeks and not guaranteed. A basement in an older house avoids the cap entirely. We usually design within the limits so the permit is not held up.
Related
Sources, as of October 2026: City of Portland, ADU zoning summary (Zoning Code Chapter 33.205); City of Gresham Development Code Section 10.0110; Lake Oswego Code LOC 50.03.004.
Updated October 2026