Mirrormont Accessory Structures: Garage Conversions Rules (2026)

A garage conversion in unincorporated King County requires a building permit under KCC Title 16, change-of-occupancy review, and if used as a habitable separate dwelling, ADU permitting under KCC Title 21A. The converted space must meet 2021 International Residential Code habitation standards: minimum ceiling height of 7 feet, at least one emergency escape and rescue opening per sleeping room, smoke alarms and carbon monoxide alarms hardwired with battery backup, one means of heating capable of reaching 68 degrees F, R-values for insulation meeting the Washington State Energy Code (ceiling R-49, walls R-21, floors R-30 typical), and egress doors. If the original garage is part of a one-hour fire-rated assembly from the dwelling, that assembly must remain intact or be replaced. Plumbing for a kitchen or bath requires Department of Health septic review if the home is on-site sewage, because adding a bedroom increases septic design flow. Zero-setback exterior walls that previously were allowed as unconditioned garage walls may need retrofitting to meet residential setback rules. Replacement off-street parking is generally not required under WA HB 1337 where the conversion is an ADU within a half-mile of transit.