ADU Rules in Pasco, Washington

Pasco (population ~82,000) is the county seat of Franklin County and one of the three Tri-Cities cities in southeastern Washington. Located at the confluence of the Columbia River and Snake River — one of the most significant river confluences in the Pacific Northwest — Pasco has a historically agricultural economy and one of Washington's largest and most established Latino communities, rooted in the farmworker population that settled in the region during agricultural expansion decades ago. Pasco is among Washington's fastest-growing cities. The city 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 Pasco. Key provisions:

• 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 Pasco to allow duplexes and middle housing in residential zones.

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

Pasco Local ADU Code

Pasco'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 within the primary structure

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

Size Limits

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

Setbacks

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

No Owner-Occupancy Requirement

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

Critical Areas Ordinance (CAO)

Pasco's CAO protects the Columbia and Snake River shorelines and riparian corridors (important habitat for salmon, steelhead, and other species migrating through the river system), wetlands, frequently flooded areas along the river corridors, and critical aquifer recharge areas. The Columbia and Snake River floodplains include FEMA-mapped flood zones. The Hanford Reach — the last free-flowing stretch of the Columbia in Washington — is adjacent to the greater Tri-Cities area. CAO buffers along river corridors can restrict ADU siting. Check Pasco's GIS mapping for CAO-regulated features on your property.

Flood Zone Considerations

Pasco's position at the confluence of two major rivers means some areas are within FEMA-mapped Special Flood Hazard Areas (SFHAs). Properties in SFHAs face building requirements including elevated foundations, which adds cost to ADU construction. Verify your property's flood zone status at msc.fema.gov before designing an ADU.

Wildfire Risk

Eastern Washington's dry, hot summers create significant wildfire risk. Properties at Pasco's urban fringe — particularly toward the Horse Heaven Hills and open sagebrush steppe — may be in Wildland-Urban Interface (WUI) zones subject to additional fire-resistant construction requirements under Washington State Building Code.

Seismic Hazard

Pasco and eastern Washington face seismic risk from the Cascadia Subduction Zone (CSZ) and local fault systems. River alluvium in the Columbia and Snake River valleys can be susceptible to liquefaction during strong earthquakes. All ADU construction must meet Washington State Building Code seismic requirements.

Permit Process

1. Check your property's zoning, flood zone, and CAO constraints at Pasco's online GIS portal
2. Contact Pasco's Community Development Department for pre-application guidance
3. Prepare construction plans meeting Washington State Building Code requirements
4. Submit permit application at pasco-wa.gov or in person at 525 N 3rd Avenue
5. Plan review and permit issuance
6. Construction inspections
7. Final inspection and Certificate of Occupancy

Contact Pasco Community Development: (509) 543-5739 | pasco-wa.gov

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