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How are ADUs Regulated in Seattle?

The City of Seattle has been actively encouraging the creation of Accessory Dwelling Units (ADUs) since 2019, making it one of the most progressive cities in the US for ADU regulations. Here's a summary:

modern rustic cottage

Key regulations:

  • Allowed in most zones: ADUs are allowed in most single-family zones (Neighborhood Residential, Residential Small Lot, and Lowrise) with some variations.

  • Number of units: Up to two ADUs per lot are permitted, either two Attached ADUs (AADUs) or one AADU and one Detached ADU (DADU).

  • Owner occupancy: You don't need to live on the property to have an ADU.

  • Parking: Off-street parking is not required for ADUs.

  • Size limits: Size limits vary depending on the zone and type of ADU. You can find detailed information on the Seattle Department of Construction and Inspection (SDCI) website.

Recent reforms:

  • In 2019, the city implemented major reforms to simplify the permitting process and reduce barriers to ADU construction.

  • This included removing the off-street parking requirement, allowing for two ADUs per lot, and streamlining the permitting process.

  • These reforms have led to a significant increase in ADU permits issued, making them a more viable housing option in Seattle.


  • The regulations can be complex, so it's recommended to consult with a professional, such as an architect, designer, or permit expeditor, to ensure your project complies with all regulations.

  • The City of Seattle offers various resources and financial assistance programs to help homeowners create ADUs. You can find more information on the SDCI and OPCD websites mentioned below.



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