ADU Rules in Kennewick, Washington

Kennewick (population ~84,000) is the largest of the Tri-Cities — the metropolitan area formed by Kennewick, Richland, and Pasco — in southeastern Washington. Located in Benton County along the Columbia River at its confluence with the Snake and Yakima Rivers, Kennewick is the commercial hub of the Tri-Cities region. The area is surrounded by Washington wine country, with the Columbia Valley AVA (American Viticultural Area) producing world-class wines. Kennewick is adjacent to the Hanford Nuclear Reservation area and is subject to Washington's statewide ADU reform laws.

Washington State ADU Law: HB 1337 (2023)

Washington HB 1337 (2023) is the foundational statewide ADU law that applies to Kennewick. Key requirements:

• Cities must allow up to 2 ADUs per lot — one attached/internal + one detached — by right on any lot with a single-family home
• No owner-occupancy requirement permitted
• No ADU-specific impact fees allowed
• Cities may not require ADUs smaller than 1,000 square feet
• State law limits how restrictive local setback, lot coverage, and height standards for ADUs can be

Washington HB 1110 (2023) accompanies HB 1337, requiring Kennewick to allow duplexes and middle housing in residential zones.

Kennewick is subject to the Washington Growth Management Act (GMA) and has a comprehensive plan with an Urban Growth Area coordinated with Benton County and the broader Tri-Cities regional planning effort.

Kennewick Local ADU Code

Kennewick's Community Development Department administers local zoning and building permits. Local ADU regulations must comply with HB 1337.

Permitted ADU Types

• Detached ADU: A standalone structure in the rear or side yard
• Attached ADU: An addition to the primary home sharing at least one wall
• Internal/Converted ADU: Conversion of existing garage, basement, or other space

Up to 2 ADUs per lot are permitted (one attached/internal + one detached).

Size Limits

State law prohibits Kennewick from requiring ADUs smaller than 1,000 square feet. Confirm current maximum size limits for your zone with Kennewick's Community Development Department.

Setbacks

HB 1337 limits Kennewick's ability to impose setbacks that would prevent reasonable ADU construction. Setback requirements vary by zoning district. Verify current standards with the Permit Center before designing.

No Owner-Occupancy Requirement

State law prohibits owner-occupancy mandates. Kennewick may not require property owners to live on-site to rent an ADU.

Hanford Nuclear Site Context

The Hanford Site is the most contaminated nuclear site in the United States, a legacy of World War II plutonium production for the Manhattan Project. Hanford is located primarily north and northwest of Richland and is managed by the US Department of Energy (DOE). While Kennewick residential properties are not part of the Hanford Site, the Columbia River downstream from Hanford — which flows past Kennewick — is subject to environmental monitoring by Washington DOE and the DOE Hanford site. Buyers and developers in the Tri-Cities area should be aware of this broader environmental context. The Hanford Reach National Monument on the Columbia River is an important conservation area adjacent to the site.

Critical Areas Ordinance (CAO)

Kennewick's CAO protects the Columbia River shoreline (including habitat for salmon and other species migrating through the system), riparian corridors, wetlands, frequently flooded areas, steep basalt slopes (Candy Mountain and other basalt ridges), and critical aquifer recharge areas. The Columbia River floodplain includes FEMA-mapped flood zones. Kennewick's ridgeline areas have significant slope considerations. Check Kennewick's GIS mapping for CAO constraints on your property.

Wildfire Risk

Eastern Washington's dry climate creates significant wildfire risk. Parts of Kennewick, particularly at the urban-rural interface on basalt ridges and sagebrush slopes, may be in WUI zones. Properties in WUI areas may face additional fire-resistant construction requirements under the Washington State Building Code.

Seismic Hazard

Kennewick and eastern Washington face seismic risk from the Cascadia Subduction Zone (CSZ) and local fault systems in eastern Washington. All ADU construction must meet Washington State Building Code seismic provisions. Basalt bedrock in higher areas provides good foundation conditions, while river alluvium in lower areas may require more careful foundation design.

Permit Process

1. Check your property's zoning and CAO status at Kennewick's online GIS portal
2. Contact Kennewick's Permit Center for pre-application guidance
3. Prepare construction plans meeting Washington State Building Code requirements
4. Submit permit application at kennewick-wa.gov or in person at 210 W 6th Avenue
5. Plan review and permit issuance
6. Construction inspections
7. Final inspection and Certificate of Occupancy

Contact Kennewick Community Development: (509) 585-4258 | kennewick-wa.gov

Disclaimer: This guide provides general information based on Washington HB 1337 (2023) and Kennewick's municipal code as of April 2026. Zoning regulations change frequently. Always verify current requirements with Kennewick's Community Development Department before making development decisions. This is not legal advice.