Olympia, WA Accessory Structures: Garage Conversions (2026)

Olympia handles garage conversions through the accessory dwelling unit standards in Section 18.04.060(A)(5) of the Olympia Municipal Code, titled Converting Existing Structures. The City of Olympia, in Thurston County, Washington, lets an ADU be created by converting an existing, legally established structure, including a detached garage, if the structure was constructed prior to December 22, 2025, or was constructed at least 10 years prior to the application for conversion. That is the unusual part of the rule: the converted building can violate current code requirements for setbacks and lot coverage and still qualify.

The existing structure can be added onto as part of the conversion, but only as long as the addition does not increase the nonconformity. The code names three examples: encroachment into a setback, the amount of impervious surface, and maximum building height. When an existing building becomes an ADU, the ADU portion of the building cannot exceed 1,000 square feet of gross floor area, unless the Director of Community Planning and Development allows a deviation under Section 18.04.060(A)(7) to use the entirety of a single floor of a dwelling constructed two or more years before the application.

A converted garage is still an ADU, so the other subsection (A) standards follow it. Up to two ADUs are allowed per lot, ADUs are subject to the lot coverage standards and minimum tree density provisions of the underlying zone, and a converted ADU is sold separately only upon approval and completion of a condominium or unit lot subdivision. The City's adopted residential code amendments in Chapter 16.04 OMC also carry a sprinkler exception for detached garage conversions to accessory dwelling units with utilities served by an existing single-family residence that does not have an existing sprinkler system.