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Accessory Dwelling Unit 
(Mother-in-Law Apartment)

See also: Garages
  • A backyard cottage is one type of mother-in-law unit.

What Is It?

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.

What Permits Do You Need?

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.

Research the Code

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:

  • An ADU is limited to 1,000 square feet in a single-family structure and up to 650 square feet in a rowhouse or townhouse
  • The ADU must meet current standards of the Seattle residential, building, mechanical, electrical, energy, land use, environmentally critical areas, and shorelines codes
  • One off-street parking space is required for the ADU except for a rowhouse or townhouse in designated urban villages and urban centers and in lowrise zones

Detached accessory dwelling unit (DADU) requirements:

  • The minimum lot size required for a DADU is 4,000 square feet in single-family zones
  • A DADU is limited to 800 square feet of gross floor area, including garage and storage areas, in single-family zones and 650 square feet in a lowrise zone
  • The DADU must meet current standards of the Seattle residential, building, mechanical, electrical, energy, environmentally critical areas, land use, and shoreline codes
  • One off-street parking space is required for the DADU, except in designated urban villages and urban centers and in lowrise zones

Should You Hire a Professional?

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.

What Do You Want To Do?


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