Kennewick, WA Accessory Structures: Garage Conversions (2026)

Garage conversions sit at the intersection of three regulatory tracks in Kennewick. First, the Washington State Residential Code at WAC 51-51 (adopting the 2021 IRC with Washington amendments) requires a building permit because converting a non-habitable garage to habitable use changes the occupancy classification and triggers full IRC compliance: R310 (emergency escape and rescue openings — minimum 5.7 sq ft net clear opening, minimum width 20 inches, minimum height 24 inches, sill height not more than 44 inches above the floor), R305 (minimum 7-foot ceiling height for habitable rooms), R303 (light and ventilation), R314 (smoke alarms in every sleeping room and adjacent to sleeping areas), R315 (carbon monoxide alarms for dwellings with fuel-fired appliances or attached garages), and R302.6 (separation between dwelling and garage). Second, KMC Title 18 requires zoning approval to confirm that the resulting use is permitted in the underlying district, floor area limits are not exceeded, and the loss of garage parking does not drop the property below the off-street parking minimum (though HB 1337 prohibits parking minimums for ADUs within one-half mile of major transit). Third, if the conversion creates a separate dwelling unit with its own kitchen and bathroom, the unit is an ADU subject to RCW 36.70A.681 and Kennewick's HB 1337 implementation. RCW 19.27.180 governs residential sprinklers: Washington has not extended automatic-sprinkler requirements to existing one- and two-family dwellings on conversion, though new construction follows IRC Appendix S where adopted. Slab-on-grade attached garages typical in Kennewick's mid-century neighborhoods around Highlands and Sunset commonly require floor leveling, insulation, and moisture-barrier upgrades to meet IRC habitability and the Washington State Energy Code (Residential) at WAC 51-11R.