Accessory Dwelling Unit

See also: Garages

City Council adopted changes to Accessory Dwelling Unit regulations that will go into effect on August 8, 2019. Check this site frequently for updates and additional information.

What Is It?

Backyard Cottage ADUAn accessory dwelling unit (ADU) 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 attached accessory dwelling unit (AADU). A legally permitted unit on the property (but not within the home) is called a detached accessory dwelling unit (DADU) or backyard cottage. Tiny houses, with foundations, are considered DADUs.

Note: Tiny houses on wheels are treated like camper trailers. You cannot live in a tiny house on wheels (or similar equipment such as RVs and boats) on lots in Seattle city limits. If your tiny house has wheels you need to follow parking rules for large vehicles.

What's New? 

The regulations for accessory dwelling units were updated by Ordinance 125854 and the majority of the requirements are effective as of August 8, 2019. The updated requirements to floor area limits in single-family zones SF5000, SF7200, and SF9600 will be effective on March 1, 2020.

Information about the new legislation is listed below: 

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 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, and in some cases in neighborhood or commercial zones. Our codes limit the size and placement of your AADU or DADU.

Requirements for any accessory dwelling units:

  • In SF5000, SF7200, and SF900 zones, up to two ADUs may be constructed. This can be either two AADUs or one AADU and one DADU. The second unit must meet specific criteria to either 1) meet green building standards or 2) be an affordable unit reserved for income-eligible households. In RSL and multifamily lowrise zones, only one accessory dwelling unit is permitted in each single-family, rowhouse, or townhouse unit. ADUs are not allowed in apartments.
  • Parking is not required for ADUs. However, you cannot remove existing required off-street parking space(s) unless replaced elsewhere on the property in a location that is allowed under the code.
  • The property owner is not required to live on the property where an ADU(s) is located.

AADU requirements:

  • An AADU is limited to 1,000 square feet in a single-family zone, including RSL, and up to 650 square feet in a lowrise zone
  • The AADU must meet current standards of the Seattle residential, building, mechanical, electrical, energy, land use, environmentally critical areas, and shorelines codes

For more information specific to AADUs see Tip 116A, Establishing an Attached Accessory Dwelling Unit (AADU).

DADU requirements:

  • The minimum lot size required for a DADU is 3,200 square feet in single-family zones
  • A DADU is limited to 1,000 square feet of gross floor area in single-family zones, including RSL, and 650 square feet in a lowrise zone
  • The DADU must meet all the Seattle residential, building, mechanical, electrical, and energy code requirements that apply to single-family dwellings

For more information specific to DADUs see Tip 116B, Establishing an Detached Accessory Dwelling Unit (DADU).

Associated Applications and Tips