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Accessory Dwelling Unit (ADU)

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 tenatitive 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 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.)

Supporting Documents:

ADU Information Sheet

ADU Types‚Äč