Thurston County Courthouse

The Thurston County Courthouse

Thurston County is proposing to improve regulations governing accessory dwelling units, or ADUs, in rural parts of the county.

Sometimes called "backyard cottages" or "mother-in-law units," an ADU is a separate living space located on the lot of an existing house that can be rented out.

ADUs are already legal within the urban growth areas of Olympia, Lacey, Tumwater, and parts of Grand Mound. However, rules vary by city, and most cities' complicated permitting and zoning processes mean that new ADUs have yet to be built in large numbers.

But in rural, unincorporated parts of Thurston County, zoning codes have not allowed for ADUs. Thurston County does allow something called a Family Member Unit in unincorporated areas, which is defined as a "temporary mobile/manufactured or modular home" which can only be occupied by a family member of the homeowner. It also must be removed when that family member leaves.

Those restrictions have made it difficult to create ADUs legally in rural areas of the county.

The family member rule also is tricky to enforce in practice, according to a staff report on the county's website.

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Many of the units that currently exist are either unpermitted structures or were permitted for different uses, according to Erin Hall, government affairs director at the Olympia Master Builders, a trade association that represents builders and remodelers in Thurston, Lewis, Mason, Grays Harbor, and Pacific counties.

It also means that existing units are less likely to have the proper septic hookups or meet other code standards.

"The Family Member Unit requirement means that currently, if you want to put an additional housing on your property in the unincorporated county, it has to be temporary," Hall said. "Structures that are temporary are going to typically be either trailers, campers, or manufactured homes of some variety, but they're built in place with the intention of not having longevity."

A code amendment proposed by the Thurston County Planning Commission would allow ADUs across the county and create designs standards regulating height, square footage, setbacks, parking, and other variables, opening the window for more permanent structures to be built.

The change would put Thurston County in line with many other counties across Washington which allow ADUs, including King, Pierce, Snohomish, Clark, and Whatcom.

A public hearing on the proposed code changes will take place via Zoom at 7 p.m. Wednesday, Aug. 5.

By November, those regulations will be submitted to the Board of County Commissioners for final approval, according to a timeline on the county's webpage.

Those who cannot attend Wednesday's meeting are invited to submit emailed comments to Associate Planner Andrew Boughan at


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