Mason County, WA Accessory Structures: ADU Rules (2026)

Section 17.03.029 of the Mason County Code governs accessory dwelling units on rural lands throughout unincorporated Mason County, WA. An ADU needs a special use permit unless it sits in shoreline jurisdiction, where a shoreline permit applies instead. It must be located on the lot of the principal residence, or created by converting an existing detached structure such as a garage. Size is capped at eighty percent of the primary residence's floor area or 1,200 square feet, whichever is smaller; if an existing home is converted into the ADU and a new primary residence is built, the cap rises to eighty percent of the new residence's area or 1,500 square feet.

Every ADU must independently meet all setback requirements and the health district's water and sewer standards, and recreational vehicles cannot count as an ADU. Only one ADU is allowed per property, and the owner must add one additional off-street parking space beyond what the primary residence requires. The rule was adopted with the 2005 development code, Ordinance 108-05, and updated by Ordinance No. 2022-006 in January 2022 and Ordinance No. 2022-093 in December 2022, which repealed and renumbered surrounding sections of Chapter 17.03.