What are Washington ADU regulations?

Washington ADU regulations vary by city. In Seattle, property owners can have two ADUs, but at least one has to be attached. Detached ADUs can be 1,000 sq ft at maximum, and in most cases, must be set back at least 20 ft from the front lot line and 5 ft from the front and rear.

In Bellevue, ADUs can be 1,200 sq ft or 40% of the primary structure, whichever is larger. They have the same front and side setback requirement as the primary structure, with a 5 ft. setback from the rear property line.