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Accessory Dwelling Unit (ADU)

​What's New:

  • December 15, 2020 -  The Board of County Commissioners adopted the Rural Accessory Dwelling Unit (ADU) ordinance at their regularly scheduled meeting on December 15, 2020 at 2pm. The following links are to PDF copies of the Notice of Adoption Legal Ad and Adopted Ordinance 15952. If you have questions on the ordinance, please contact 

Project Description

Thurston County is considering an amendment to code language to allow a small, separate living unit on the same lot as a single-family home in rural, unincorporated parts of the County.  Design standards for these accessory dwelling units (ADUs) are being explored as part of the Development Code Docket.  

An Accessory Dwelling Unit (ADU) is a small, separate living unit built on the same lot as a single-family home. Traditionally, ADUs can be an addition to a single-family home, a conversion of a basement or attached garage, or a freestanding "detached" structure. An ADU has all the basic facilities for day-to-day living, such as a kitchen, sleeping area, and bathroom. As the term "accessory" implies, ADUs are smaller in size, use and prominence than the main residence on the lot.

Project Timeline


**The project timeline is tentative and subject to change. 


Thurston County currently does not allow ADUs in the rural county. However, ADUs are allowed within the urban growth areas (UGAs) of Olympia, Lacey and Tumwater. ADUs are also allowed in certain zones of the Grand Mound UGA. 

While Thurston County does not currently allow ADUs, the county does allow Family Member Units (FMUs). Currently, a FMU must be a temporary mobile/manufactured or modular home. Only family members may live in FMUs, and the FMU must be removed once the family member leaves.

The proposed policy for ADUs would change design and development standards (i.e., setbacks, building height, etc.)

Public Participation Opportunity 

Thank you for your interest in the Rural Accessory Dwelling Unit (ADU) docket item. If you would like to participate in the public process, please click Comment on the Accessory Dwelling Unit (ADU) to provide comments on this proposal.

ADU Types​