An accessory dwelling unit is a second, smaller home on a lot that already has one: a detached cottage in the back yard, a garage or outbuilding converted to living space, or a unit attached to the main house.
Washington opened this up considerably. State law now requires cities planning under the Growth Management Act to allow up to two ADUs per residential lot inside urban growth areas, with no owner-occupancy requirement, and a 2026 change added a pathway for one detached ADU on rural parcels in counties that opt in. Local setbacks, height, design and stormwater rules still apply, and an HOA covenant can still restrict what the city would allow — exactly the kind of thing worth checking before you spend money on drawings.
We handle that check, the permits and the build. People build these for aging parents, for adult kids, for rental income, or to live in while the main house gets renovated. The reason changes what makes sense to build, so it is the first thing we ask.