Accessory Structures in Mason County, WA (2026)
3 rules for unincorporated Mason County, Washington.
Verified from official government sources
ADU Rules
Mason County Code § 17.03.029 caps an accessory dwelling unit in unincorporated Mason County's rural lands at eighty percent of the primary residence's floor area or 1,200 square feet, whichever is smaller, limits every parcel to one ADU, and requires an extra off-street parking space. The ADU must sit on the lot of, or convert an existing detached structure near, the main house.
Accessory Dwelling Unit Rules in Mason County, WA
Some RestrictionsMason County Code § 17.03.029
In rural lands, accessory dwelling units (ADU) must meet the following requirements: ... (3)The ADU shall be located on the lot of the principal residence or shall be a conversion of an existing detached structure (i.e. garage);(4)The ADU shall not exceed eighty percent of the area of the primary residence or one thousand two hundred square feet, whichever is smaller. An existing residence can ...
ADU Owner Occupancy
In shoreline jurisdiction within unincorporated Mason County, WA, Mason County Code § 17.03.029(2) requires the owner of an accessory dwelling unit to live on the lot, in either the main house or the ADU itself. Outside shoreline jurisdiction, the county's general ADU standards apply without that residency requirement.
ADU Owner-Occupancy Rule in Mason County, WA
Some RestrictionsMason County Code § 17.03.029(2)
In rural lands, accessory dwelling units (ADU) must meet the following requirements: (1)The ADU shall be subject to a special use permit, unless in the shoreline jurisdiction, it is subject to a shoreline permit;(2)In shoreline jurisdiction, the owner of the ADU must reside on the lot in either the principal residence or ADU;
Shed & Carport Rules
A storage shed in unincorporated Mason County skips the building permit only if it is single-story, five hundred seventy-six square feet or smaller, no taller than ten feet at the wall, and set back at least ten feet from other structures, per Mason County Code Section 14.08.031(2). Larger or taller sheds need a permit.
Mason County Shed and Accessory Building Rules
Some RestrictionsMason County Code § 14.08.031(2)
Sections [A]105.2 and R 105.2 of the International Building Code and International Residential Code are adopted herein by reference and supplemented with the following: ... (2)Storage (Non-Agricultural) Buildings: Single-story buildings or structures used for the storage of belongings, not designed for human habitation, and not used for remunerative purposes. These structures shall not be a pla...
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