Thurston County, WA Accessory Structures: Garage Conversions (2026)
Garage conversion rules in Thurston County, WA, sometimes called garage-to-ADU or accessory living unit conversions, govern permits, ceiling height, egress, and parking replacement.
Key Facts
- Size limit for conversions
- None, if the work stays within the existing footprint
- Parking
- Displaced parking replaced; additional parking under Chapter 20.44.030
- Water
- Water and sanitation required; COWA for Group A or B systems
- ADUs per parcel (rural)
- One, with a detached single-family dwelling
- Density
- A conversion does not count as a dwelling unit
- Largest new detached ADU
- 1,000 sq ft, or 1,200 sq ft with one TDR
- Rule last amended
- Ord. No. 16575, December 16, 2025
Summary
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.
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 detached space to an ADU, so long as the conversion remains within the footprint of the structure. ... Conversion: Displaced parking must be replaced and additional parking must be provided subject to Chapter 20.44.030 , Off-street parking, required spaces. Modifications or expansions of the converted structure shall be subject to the design standards in the subject zoning district. ... Accessory dwelling units must have water and sanitation. For any ADU connecting to a Group A or Group B water system, a certificate of water availability (COWA) is required and shall also comply with Section 14.48.100 (B)10—12. This provision applies to all ADUs, including those that may not require a building permit (e.g., internal conversions).
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Code of Ordinances: Supplement 77).
Full Breakdown
Thurston County Code § 20.34.020 governs rural accessory dwelling units, and its conversion provisions are the rules that apply when a garage, shop or similar building becomes a dwelling. The section applies in every Title 20 zoning district that allows residential uses, except R 5-12/1, R 8-24/1 and AC districts and the residential districts inside the urban growth areas of Yelm, Rainier and Tenino. Those excluded areas follow the urban ADU section, § 20.34.040.
The county allows one ADU per residential parcel, tied to a detached single-family dwelling, and an ADU may not be placed on a parcel that has any other accessory residential structure. A conversion is not counted as a dwelling unit when density is calculated, so no land area beyond the zone's minimum parcel size for the single-family home is required.
Size is the most generous part of the conversion rule. Subsection 4(b) says there is no limit on size for converting existing internal or detached space, so long as the conversion remains within the footprint of the structure. A new detached ADU, by contrast, is capped at 1,000 square feet, or 1,200 square feet with one transferable development right purchased from the Long-Term Agriculture District under Chapter 20.62.
Parking is the main condition. Under subsection 5(b), displaced parking must be replaced and additional parking must be provided under Chapter 20.44.030, off-street parking, required spaces. Any later modification or expansion of the converted structure must meet the design standards of the zoning district.
Water and sanitation apply even when no building permit is needed. Subsection 6(a) requires every ADU to have water and sanitation, and an ADU connecting to a Group A or Group B water system needs a certificate of water availability (COWA) that also complies with Section 14.48.100(B)10 through 12. The code states this applies to internal conversions. The ADU must also share the existing driveway to the public right-of-way, hard-surface coverage limits still apply, and the building must meet Title 14 and all other applicable codes. The provisions were last amended by Ord. No. 16575 on December 16, 2025.
Violations & Fines
Section 20.34.020 prints no penalty amount of its own. Subsection 10 requires every ADU to conform with Title 14, Buildings and Construction, and all other applicable codes and ordinances, so a garage turned into living space without replacing the displaced parking, securing a COWA where a Group A or B system serves it, or meeting Title 14 stands outside the code. Any fine or correction order comes from those other provisions.
Frequently Asked Questions
Is there a maximum size for a garage converted to an ADU in rural Thurston County?
Do I have to replace the parking spaces the garage provided?
Does the conversion count against my parcel's density?
Do I need a water availability certificate if no building permit is required?
Do these rules apply inside Yelm, Rainier or Tenino?
Sources & Official References
Other rules in Thurston County
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