Mason County, WA Accessory Structures: ADU Owner Occupancy (2026)

Mason County Code § 17.03.029 sets nine requirements for accessory dwelling units on rural lands, and subsection (2) adds an owner-occupancy condition that applies specifically inside shoreline jurisdiction: the owner of the ADU must reside on the lot, in either the principal residence or the ADU itself. That residency rule pairs with subsection (1), which routes an ADU in shoreline jurisdiction to a shoreline permit rather than the special use permit that governs ADUs elsewhere in the rural lands. The distinction exists because Mason County's shoreline master program, Chapter 17.50, applies heightened review along Hood Canal, Lake Cushman and other shoreline areas of the county, and the county uses owner-occupancy to keep ADUs there tied to genuine residential use rather than investment rental stock.

An ADU outside shoreline jurisdiction is not subject to subsection (2) and instead follows the special use permit path and the other numbered standards in § 17.03.029, including the one-ADU-per-lot limit, the size cap tied to the primary residence, and the added off-street parking space. Community Development staff confirm whether a parcel falls in shoreline jurisdiction, mapped under the county's GIS layers referenced in MCC 17.02.060, before applying the owner-occupancy condition to a shoreline ADU application.