ADU Rules in Auburn, Washington

Auburn (population ~90,000) is a diverse, growing city spanning the King County and Pierce County border in the White River valley south of Seattle. Located between the Green and White Rivers, Auburn has a manufacturing and logistics heritage and an increasingly diverse population. The Green River Trail and access to the scenic Green River Gorge make Auburn an attractive outdoor recreation base. Auburn 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 Auburn. Core 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 Auburn to allow duplexes and middle housing in residential zones.

Auburn is subject to the Washington Growth Management Act (GMA) and coordinates Urban Growth Area planning with both King and Pierce Counties, given its unique dual-county location.

Auburn Local ADU Code

Auburn's Planning and 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 Auburn from requiring ADUs smaller than 1,000 square feet. Confirm current maximum size limits for your zone with Auburn's Planning Department.

Setbacks

HB 1337 limits Auburn'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. Auburn may not require property owners to live on-site to rent an ADU.

Critical Areas Ordinance (CAO) and Flood Zones

Auburn's position in the White River valley creates significant critical areas considerations:

• White River: An important salmon-bearing river (Chinook, Coho, Steelhead) with a wide floodplain and CAO buffers. The White River flows from the Green River downstream of Auburn's valley
• Green River: Also a salmon-bearing river with floodplain, buffers, and habitat protections through and near Auburn
• FEMA Flood Zones: Substantial areas of Auburn's valley floor are in FEMA-mapped Special Flood Hazard Areas (SFHAs). Properties in SFHAs face elevated foundation requirements that add cost and complexity to ADU construction
• Wetlands: Numerous wetlands in the river valleys require CAO buffers

Check Auburn's GIS mapping and FEMA flood maps at msc.fema.gov before designing an ADU on valley-floor property.

Seismic Hazard

Auburn and the Puget Sound region face significant earthquake risk from the Cascadia Subduction Zone (CSZ). Auburn's White River and Green River valley floors have alluvial sediments susceptible to liquefaction during strong shaking. USGS and Washington DNR seismic hazard maps identify high-risk areas. ADU foundations on valley-floor lots should incorporate geotechnical assessment. All construction must meet Washington State Building Code seismic requirements.

Permit Process

1. Check your property's zoning, flood zone, and CAO constraints at Auburn's online GIS portal
2. Contact Auburn's Permit Center for pre-application guidance
3. Prepare construction plans meeting Washington State Building Code requirements (including any flood zone elevations)
4. Submit permit application at auburnwa.gov/permits or in person at 1 East Main Street
5. Plan review and permit issuance
6. Construction inspections
7. Final inspection and Certificate of Occupancy

Contact Auburn Permit Center: (253) 931-3020 | auburnwa.gov/permits

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