Accessory Structures in Thurston County, WA (2026)
3 rules for unincorporated Thurston County, Washington.
Verified from official government sources
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 RestrictionsThurston 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...
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 RestrictionsThurston 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...
Garage Conversions
In rural unincorporated Thurston County, converting an existing garage or other internal or detached space into an accessory dwelling unit has no size cap as long as the work stays within the structure's footprint. Displaced parking must be replaced, a certificate of water availability may be needed, and the unit does not count toward density.
Thurston County Garage and Outbuilding Conversion to ADU Rules
Some RestrictionsThurston County Code § 20.34.020(2)(b), (4)(b), (5)(b), (6)(a)
An ADU conversion, as defined under this title, does not count as a dwelling unit for the purpose of calculating density. No additional land area is required for such ADUs beyond the minimum parcel size that is required for the associated single-family dwelling structure under the applicable zoning district. ... Conversion: There is no limit on size for the conversion of existing internal or de...
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