How big can an ADU be in Lake Oswego?
As of October 2026, the Lake Oswego Code (LOC 50.03.004) limits an ADU to 800 sq ft of gross floor area, with an exception for some conversions of existing floor area. Gross floor area is measured differently from Portland's "living area," so the same unit can count differently in each city.
How many ADUs can I have?
One. The code allows no more than one additional unit with a single-family dwelling. In the R-5, R-3, R-2 and R-0 zones the ADU must go with a detached single-family house. The city's residential use table does not list ADUs as allowed in the R-W zone.
Where can the entrance go?
- Interior conversions: the ADU's separate entrance may not be on the front facade of the existing house, and no variance to that rule is allowed.
- Detached ADUs close to a property line: when the unit is less than ten feet from a line, its entrance must be at least ten feet from any property line.
The code allows minor variances to some ADU design standards, but not major variances, or design variances made only to locate an ADU.
What else shapes a Lake Oswego project?
Slopes, mature trees and lake-area lots can all add review or engineering before construction. We look at grades, tree locations and access for materials on the site visit.
Conversion or new cottage?
It depends on the house. Converting existing floor area can qualify for the code's exception to the 800 sq ft cap, while a new detached unit cannot exceed it. On a steep or heavily treed lot, converting space inside the house may also avoid site work. We compare both paths.
Related
As of October 2026. Sources: Lake Oswego Code Chapter 50 (LOC 50.03.004 and the residential use table); ORS 197A.425.
Check what fits on your Lake Oswego lot
Send your address. We check the zone, the size rule and where an entrance can go.