Accessory Dwelling Units (ADUs)

Attached ADU in Issaquah (Photo Credit: Brett VA)
Attached ADU in Issaquah (Photo Credit: Brett VA)

Accessory Dwelling Units (ADUs) are self-contained residential units located on the same lot as an existing residential property. Whether built concurrent to the primary residence or at a later time, they can be located within the primary residence (ex: complete basement apartment with separate entrance), attached to it, or in a separate structure. ADUs can help add more affordable housing and density without dramatically changing neighborhood character. In addition, they create flexibility within the existing housing stock and options for multigenerational households.

To further encourage ADUs, some communities have developed libraries of pre-approved plans for ADUs for people to purchase and build. Because the plans are standardized and pre-approved for permitting, building an ADU becomes much simpler and more affordable. This article from MRSC provides more detail on this subject, including a number of example programs from Portland, Santa Cruz, and more.

Detached ADU over garage in Denver (Photo Credit: Brett VA)
Detached ADU over garage in Denver (Photo Credit: Brett VA)

Per RCW 43.63A.215, Washington cities and towns with populations above 20,000 residents are required to plan for ADUs in single family neighborhoods.

Where it works

ADUs can work in all communities, particularly those that are single family-dominant and/or built out. Certain restrictions commonly used, including owner occupancy requirements and excessive parking minimums, can limit the success of this housing type.

Performance measures

  • Number of units built

More information

Areas for further research

  • How common illegal units are by community
  • How local ADU rents compare to other forms of housing


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