Accessory Structures in Thurston County, WA (2026)

3 rules for unincorporated Thurston County, Washington.

Verified from official government sources

Note: These county rules apply to unincorporated areas of Thurston County. If you live within an incorporated city, that city's own rules apply: see the city list below.

ADU Rules

Thurston County caps accessory dwelling units at 850 square feet and allows only one ADU per lot with a single-family home.

Thurston County Accessory Dwelling Unit Rules

Some Restrictions

Thurston County Code § 21.06.055

"Accessory dwelling" means a separate living unit (apartment) integrated within a single-family dwelling, or one located as a detached accessory dwelling located on the same lot as a single-family dwelling. Accessory dwellings shall be limited to eight hundred fifty square feet in floor area and, for the purposes of calculating residential density, shall not count as a dwelling unit. There shal...

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Shed & Carport Rules

In Thurston County's high-density residential zoning, a one-story accessory building under 200 square feet that's exempt from a building permit gets reduced setbacks: 10 feet front, 3 feet side, 5 feet rear.

Thurston County Shed Setback Rules

Some Restrictions

Thurston County Code § 21.18.040(C)(5)

5.Accessory Buildings. All accessory buildings must comply with the current buildings setbacks as stated in this chapter; provided, however, if the accessory building is exempt from obtaining a building permit under Title 14 TCC, one-story, and is less than two hundred square feet, the following setbacks are permitted: a.Front yard, ten feet, b.Side yard, three feet, c.Rear yard, five feet (thr...

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Garage Conversions

Thurston County lets homeowners turn an existing garage or other internal space into an accessory dwelling unit under Code Section 20.34.020, and unlike an attached or detached addition, a garage conversion carries no square-footage cap so long as the finished unit stays inside the original structure's footprint. The county still requires replacement parking and water/sanitation compliance before occupancy.

Thurston County Garage-to-ADU Conversion Rules

Some Restrictions

Thurston County Code § 20.34.020(4)(b), (5)(b), (6)(a)

Accessory dwelling units (ADU) are permitted in all Title 20 zoning districts that allow for residential uses subject to the following requirements: ... 4.Size Limitations. ... b.Conversion: There is no limit on size for the conversion of existing internal or detached space to an ADU, so long as the conversion remains within the footprint of the structure. ... 5.Specific Standards. ... b.Conver...

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