Thurston County, Washington

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Thurston County Washington

Community Planning and Economic Development

ADU (Accessory Dwelling Unit) Information

The Board of County Commissioners took final action on Thurston 2045 and other 2025 Comprehensive Plan Amendments and adopted Resolution 16574 and Ordinance 16575 on December 16, 2025. The changes include amendments to Accessory Dwelling Units.

  • Family Member Units are no longer permitted.
  • One ADU per lot is permitted in the rural area.
  • Detached ADUs are permitted in the rural area and do not require additional land.
    • Size cap of 1,000 square feet. Transferable development rights may be used to increase the size to 1,200 square feet.
    • Must be within 100 ft from the primary residence, unless there are limiting circumstances.
    • Must share the primary driveway.

Accessory Dwelling Units (ADUs) are smaller homes on the same lot as a single-family home. It can be attached like an apartment over a garage or detached like a backyard cottage. An ADU includes a kitchen, bedroom(s), and bathroom. Thurston County allows ADUs in rural unincorporated areas of Thurston County when density requirements for the underlying zoning district, regulations, and standards pertaining to ADUs can be met.  

Find forms and details on the County's permitting applications web page under the ADU at the top. 

Rural Accessory Dwelling Units

Illustrated diagram showing a primary home and a detached accessory dwelling unit (ADU) connected by one shared driveway. A dashed arc and measuring tape graphic indicate the ADU must be within 100 feet of the main house (“<100 feet max”). Labels identify “Primary Home,” “Detached ADU,” “One Shared Driveway,” and the street in front. Trees and landscaping surround both buildings.

Where Rural ADUs Are Allowed

All rural zones that allow residential uses.

Urban ADUs are allowed in Urban Growth Areas (UGAs) and are subject to different rules. See section below.

How Many ADUs?

One ADU is allowed per lot.

Size Rules

Attached ADUs

  • Must be no more than 50% of the size of the main house.

Detached ADUs

  • Must be 1,000 square feet or less. (1,200 square feet if a transferable development right is used.)
  • Must be located within 100 feet of the main house.

Water & Septic Requirements

  • Have approved water and septic.
  • Provide a Certificate of Water Availability (COWA).
  • Meet County code 14.48.100(B).

 

Urban Accessory Dwelling Units

Where Urban ADUs Are Allowed

Up to two ADUs per lot in the following UGAs:

  • Grand Mound, Yelm, Rainier, Tenino, Lacey, Tumwater, and Olympia

The ADUs can be:

  • One attached accessory dwelling unit and one detached accessory dwelling unit;
  • Two attached accessory dwelling units; or
  • Two detached accessory dwelling units, which may be comprised of either one or two detached structures.

Size Rules

  • Grand Mound, Yelm, Rainier, and Tenino: 1,000 square feet or less.
  • Lacey: 850 square feet or less.
  • Tumwater: 300-800 square feet.
  • Olympia: 800 square feet or less.

Water & Septic Requirements

  • Have approved water and septic OR connection to water and sewer.
  • Provide a Certificate of Water Availability (COWA).
  • Meet County code 14.48.100(B).

Other Requirements

 

Additional Information

When the County adopts new resolutions or codes, it can take up to six months before they are shown in Municode. The following codes have been adopted but as of December 16, 2025 are not yet reflected in Municode. Please refer to Ordinance 16575 for the ADU rules.

Detailed rules: Chapter 20.34 of Thurston County code. 

Other regulations may also apply, including:

  • Shoreline Master Program (SMP)
  • Critical Areas Ordinance (CAO)
  • Health Code