An accessory dwelling unit or detached accessory dwelling unit (sometimes called a mother-in-law apartment) is a separate living space within a house or on the same property as an existing house. These units aren’t legal unless they have been established through a permit process. A legally permitted unit in the home is called an accessory dwelling unit (ADU). A legally permitted unit on the property (but not within the home) is called a backyard cottage or detached accessory dwelling unit (DADU). The property owner must live in either the house or the attached or detached accessory dwelling unit.
Adding within an existing house. You need a construction addition / alteration permit.
Building a detached unit. You need a construction new addition / alteration permit.
Legalizing an existing unit. You need a construction permit to establish use.
You may also need to apply for electrical service changes or new services from Seattle City Light.
You can build these separate living spaces in a single-family or lowrise zone. Our codes limit the size and placement of your accessory or detached unit. The owner is required to live in either the house or in the additional unit; we require a signed owner occupancy covenant agreeing to this condition.
Attached accessory dwelling unit (ADU) requirements:
Detached accessory dwelling unit (DADU) requirements:
We recommend that you hire a professional to assist in the design process. Accessory or detached dwelling units that fall under the Seattle residential code do not require a professional stamp indicating they were created by an architect or engineer.