Kennewick, WA Accessory Structures: ADU Rules (2026)

Washington's HB 1337 (2023) at RCW 36.70A.681 requires cities and counties planning under the Growth Management Act to allow two ADUs per residential lot — for example, one detached and one attached, or two detached — in any area zoned predominantly residential. The statute preempts a long list of restrictive local rules: under RCW 36.70A.681(2) cities may not impose owner-occupancy requirements (codified separately at RCW 36.70A.683); may not require off-street parking for ADUs within one-half mile of a major transit stop; must allow ADUs of at least 1,000 square feet; may not require an ADU to match the architectural design of the principal dwelling; and may not impose setbacks more restrictive than those for the principal dwelling. RCW 36.70A.696 caps ADU impact fees at 50 percent of the fee charged for an equivalent single-family residence. Kennewick implements these mandates through KMC Title 18 (Zoning), with the substantive ADU provisions typically in KMC Chapter 18.42 or a successor chapter (Accessory Dwelling Units). Construction must additionally comply with the Washington State Building Code Council's adoption of the International Residential Code at WAC 51-51 and the Washington State Energy Code (Residential) at WAC 51-11R. Kennewick lies within Mid-Columbia seismic Zone D1 and the high-wind exposure typical of the Columbia Basin; structural design must reflect those parameters.