Accessory Structures in Kennewick, WA: What Residents Actually Need to Know

If you live in Kennewick or are thinking about moving there, accessory structures are one of those things you probably won't think about until they affect you directly. Kennewick has 5 specific rules on the books covering different aspects of accessory structures, and some of them might surprise you.

ADU Rules

Kennewick (Benton County, population approximately 84,000) is a Washington city subject to the state's 2023 ADU preemption statute, House Bill 1337 codified at RCW 36.70A.680 through RCW 36.70A.700. Because Kennewick exceeds the 25,000-population threshold and lies within a fully-planning GMA county (Benton County), it must permit two accessory dwelling units (ADUs) per lot in all areas zoned predominantly for residential use. Local detail is implemented through Title 18 of the Kennewick Municipal Code (Zoning), hosted on Code Publishing at https://www.codepublishing.com/WA/Kennewick/.

Key details: State ADU Preemption: RCW 36.70A.681 (HB 1337, 2023). Units Per Lot: 2 ADUs required by state law. Owner-Occupancy: Banned (RCW 36.70A.683). Local Code: KMC Title 18 (Zoning). Building Code: WAC 51-51 (2021 IRC).

Constructing or occupying an ADU without permits violates KMC Title 18 and the Washington State Building Code as adopted by Kennewick under RCW 19.27. Enforcement under KMC Title 1 (General Provisions) and Chapter 1.04 (Code Enforcement) includes notices of violation, civil penalties, stop-work orders from the Kennewick Building Division, and abatement actions in Benton County Superior Court. Local rules that conflict with HB 1337 are preempted under RCW 36.70A.681(2) and unenforceable; the Washington Attorney General and the Department of Commerce review GMA compliance. Continued violations may trigger daily fines under KMC enforcement provisions.

The rules around adu rules in Kennewick lean permissive, but that does not mean anything goes.

Shed Rules

Sheds in Kennewick are regulated through two layers: (1) Kennewick Municipal Code Title 18 (Zoning) setting accessory-structure dimensional standards by district — typically rear-yard location, district-specific setbacks, and a maximum height around 15 feet; and (2) the Washington State Building Code at WAC 51-51, adopting the 2021 International Residential Code, which under IRC R105.2 exempts one-story detached accessory structures of 200 square feet or less from building permit requirements but does not waive zoning compliance. A zoning permit from the Kennewick Planning Division is generally still required even when a building permit is not.

Key details: WA Building Permit Exemption: ≤200 sq ft (IRC R105.2 / WAC 51-51). Zoning Clearance: Generally still required. Typical Location: Rear yard, behind front building line. Accessory Height: ~15 ft (KMC Title 18 district-specific). Ground Snow Load: ~15-25 psf (Mid-Columbia).

Installing a shed without a required zoning clearance violates KMC Title 18, enforced under KMC Chapter 1.04 (Code Enforcement) through notices of violation, civil penalties, and abatement orders. Unpermitted sheds in setbacks or exceeding lot coverage may be ordered relocated or removed. Sheds over 200 square feet built without a building permit additionally violate the Washington State Building Code under RCW 19.27 and the Kennewick adoption of the IRC, triggering stop-work orders from the Building Division and after-the-fact permit fees commonly at double the standard rate.

Garage Conversions

Converting a Kennewick garage into habitable space requires both (1) zoning approval under KMC Title 18 for the change of use, since the converted area no longer functions as accessory parking and may trigger off-street parking minimums or ADU classification under HB 1337; and (2) a building permit under the Washington State Building Code at WAC 51-51 (2021 IRC). Conversions must meet IRC Chapter 3 requirements for habitable space including R310 emergency escape and rescue openings, R305 ceiling height, R314 smoke alarms, and R315 carbon monoxide alarms. RCW 19.27.180 governs residential sprinkler requirements; one- and two-family dwellings are generally exempt from automatic sprinkler retrofit on conversion.

Key details: Building Code: WAC 51-51 (2021 IRC). Egress Standard: IRC R310 (5.7 sq ft minimum). Ceiling Height: IRC R305 (7 ft habitable rooms). Sprinkler Statute: RCW 19.27.180 (1-2 family exempt). Zoning Review: Change-of-use approval (KMC Title 18).

Performing a garage conversion without permits violates the Washington State Building Code under RCW 19.27 and KMC Title 18. Enforcement includes stop-work orders from the Kennewick Building Division, requirement to undo the conversion or obtain after-the-fact permits at elevated fees under KMC Chapter 1.04, and civil action in Benton County Superior Court. Unpermitted habitable conversions that fail to meet IRC R310 egress are commonly cited in fire-injury cases and may expose the owner to liability and insurance denial. Failure to install CO alarms violates R315 as adopted by Washington.

ADU Permits

An ADU in Kennewick requires permits from two municipal tracks: a zoning clearance confirming the ADU complies with KMC Title 18 as updated for HB 1337 compliance, and a building permit under the Washington State Building Code at WAC 51-51. Because Washington HB 1337 (RCW 36.70A.681) preempts discretionary review for compliant ADUs, Kennewick must process most ADU applications ministerially within the time frames imposed by RCW 36.70B.080 (local project review). The Kennewick Planning Division and Building Division handle the application. The Kennewick Municipal Code is on Code Publishing at https://www.codepublishing.com/WA/Kennewick/.

Key details: Permit Tracks: Zoning + Building (both required). Review Type: Ministerial under HB 1337. Processing Standard: 120 days (RCW 36.70B.080). Building Code: WAC 51-51 (2021 IRC). Local Code: KMC Title 18.

Constructing an ADU without permits violates the Washington State Building Code under RCW 19.27 and KMC Title 18. Enforcement is through notice of violation, civil penalties, and stop-work orders from the Kennewick Building Division under KMC Chapter 1.04 (Code Enforcement). After-the-fact permits typically carry doubled fees and require the applicant to demonstrate code compliance through invasive inspections. Unpermitted occupancy without a Certificate of Occupancy is also a violation. Permit conditions that conflict with HB 1337 — for example, owner-occupancy requirements (RCW 36.70A.683), unit-size caps below 1,000 square feet, or parking minimums within one-half mile of major transit — are preempted by state law and unenforceable.

The rules around adu permits in Kennewick lean permissive, but that does not mean anything goes.

ADU Impact Fees

Washington HB 1337 at RCW 36.70A.696 caps impact fees for accessory dwelling units at 50 percent of the impact fee charged for an equivalent single-family residence. The underlying GMA impact-fee authority is at RCW 82.02.050 through 82.02.110, which authorizes counties, cities, and towns planning under the Growth Management Act to impose impact fees for public streets and roads, parks and recreation, schools, and fire protection. Kennewick (subject to the GMA in Benton County) may charge transportation, parks, and school impact fees through KMC and the local fee schedule, but all ADU fees are subject to the 50 percent statutory cap. School impact fees flow to the Kennewick School District under interlocal agreement.

Key details: ADU Impact Fee Cap: 50% of SFR (RCW 36.70A.696). Authorizing Statute: RCW 82.02.050-.110. Allowed Categories: Roads, parks, schools, fire. School Fees: Kennewick School District (interlocal). Connection Fees: RCW 35.92.025 (separate from impact fees).

Charging an ADU impact fee in excess of 50 percent of the single-family fee violates RCW 36.70A.696 and is subject to refund. Failure to pay required impact fees authorized under RCW 82.02 prevents permit issuance or Certificate of Occupancy under KMC and the local fee ordinance. Misapplication of impact fees to ineligible categories (e.g., library impact fees, which are not authorized for GMA impact fees) is challengeable under RCW 82.02.070. School districts that receive impact fees must spend or refund within 10 years of receipt under RCW 82.02.070(3). Applicants who believe an impact fee is wrongly imposed may seek administrative appeal under KMC and judicial review under the Land Use Petition Act (RCW 36.70C).

Kennewick is more permissive than most cities when it comes to adu impact fees. That said, there are still limits.

The Bottom Line

Compared to many U.S. cities, Kennewick gives residents more room on accessory structures. 3 of the 5 rules here are rated permissive. But permissive does not mean unregulated. There are still requirements, and the city does enforce them when violations are reported.

These rules come from Kennewick's publicly available municipal code. For complete penalty schedules, exemption details, and answers to common questions, see the individual ordinance pages throughout this guide.