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Accessory Dwelling Unit (ADU)
The City has recently updated its regulations for construction of Accessory Dwelling Units (ADUs). An ADU, sometimes informally referred to as a “mother-in-law apartment,” can be built in residential areas with minimal impact to existing neighborhoods. They can also provide a source of extra income to home owners, sometimes making it easier for elderly residents to stay in their homes.
The new ADU regulations remove some of the restrictions in
the old rules, and include the following:
- ADUs can be within, attached to, or detached from to the primary unit. For a minimum of nine months in any twelve month period, the owner of the primary unit must live in either the primary unit or the Accessory Dwelling Unit;
- Only one ADU is allowed per single-family structure;
- All ADUs need to be at least 220 sq. ft. in area. The maximum size of the ADU will vary depending upon the type built. Detached ADUs, those created through an addition, and those designed into the structure at the time of permitting can be up to 800 sq. ft. An ADU created within an existing structure can be up to 45% of the total area of the primary unit;
- A minimum of one off-street space is required for an ADU.
All new and existing ADUs must register with the City. New ADUs can be registered during the permitting process, while existing ADUs have twelve months to register and bring their units up to the current standards if they were built without permits.
A Building Permit application must be completed for all new accessory dwelling units, along with an ADU Registration form and Affidavit. If your ADU will be completely within the existing main home, you must also fill out an Residential Remodel Checklist. If the ADU will be attached to the main home, or completely detached from it, fill out the New/Addition Residential Checklist.
For more information, call the Department of Planning & Community Development at 206.973.4840.
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City of SeaTac Department of Planning and Community Development
4800 South 188th Street, SeaTac, WA 98188 206.973.4830
© 2001 City of SeaTac. All rights reserved
Last Updated: 02/10/05