Washington ADU permit requirements
Last reviewed: June 20, 2026
Quick answer
Permitting an accessory dwelling unit (ADU) in Washington means pulling a building permit from the local authority that issues it. Washington has a statewide ADU law that sets baseline standards local agencies must follow. ArchPermit tracks 15 source-cited requirements (11 required, the rest conditional) for this jurisdiction. These requirements were last reviewed June 20, 2026; confirm the exact forms and fees with the local permitting authority.
Washington has a statewide ADU law. This baseline reflects that law plus the universal new-dwelling deliverables. Your local city or county still administers the permit and may add its own forms and fees — confirm the exact checklist with them.
Washington's EHB 1337 (2023) mandates ADUs in GMA urban growth areas, but the local city/county is the permitting AHJ and administers the code. Implementation phases in with each jurisdiction's comprehensive-plan update — confirm local rules.
- Adopted building code compliancerequired
The ADU must comply with Washington's adopted residential building code: 2021 IRC. Confirm the in-force edition with the local AHJ.
source · reviewed 2026-06-20 - Building cross-sectionsrequired
Cross-sections through the structure showing ceiling heights, wall/roof assemblies, insulation, and fire/sound-rated assemblies where required.
source · reviewed 2026-06-20 - Egress & life-safetyrequired
Emergency escape and rescue openings (egress windows), smoke and carbon-monoxide alarms, and required exits per the adopted residential code.
source · reviewed 2026-06-20 - Energy code compliancerequired
Energy-code compliance documentation for the conditioned space. Washington: 2021 Washington State Energy Code (WSEC-R).
source · reviewed 2026-06-20 - Exterior elevationsrequired
Elevations of each affected building face showing height, exterior materials, and openings; existing vs. proposed where exterior changes occur.
source · reviewed 2026-06-20 - Floor plansrequired
Dimensioned floor plans of the ADU (and affected areas of the primary dwelling) showing rooms, uses, door/window locations, egress, and fixture/equipment locations.
source · reviewed 2026-06-20 - Foundation planrequired
Foundation plan with footing/stem-wall sizes and details (or a cited site-specific design), per the adopted residential code.
source · reviewed 2026-06-20 - Framing / structural plansrequired
Floor, wall, and roof framing showing member sizes, spans, spacing, connections, and lateral bracing; structural calculations where the framing is non-conventional.
source · reviewed 2026-06-20 - Local zoning / setback compliancerequiredverify locally
Compliance with the local jurisdiction's zoning: permitted use, setbacks, height, lot coverage, and any ADU-specific standards. These are set locally — confirm with the permitting agency.
source · reviewed 2026-06-20 - Mechanical / plumbing / electricalrequired
HVAC, plumbing fixtures and lines, and electrical (panels, circuits, outlets, lighting, smoke/CO alarms) for the new unit.
source · reviewed 2026-06-20 - Site / plot plan (to scale)required
Scaled site plan showing property lines, setbacks, existing structures, the proposed ADU footprint with dimensions to property lines, driveways/parking, and utility locations.
source · reviewed 2026-06-20 - ADUs under 1,000 sq ft cannot be prohibitedconditional
A size cap below 1,000 gross sq ft may not be imposed.
source · reviewed 2026-06-20 - No ADU parking within ½ mile of major transitconditionalverify locally
No on-site parking may be required for ADUs within ½ mile of a major transit stop.
source · reviewed 2026-06-20 - No owner-occupancy requirementconditional
The jurisdiction may not require the owner to occupy the principal unit or any ADU.
source · reviewed 2026-06-20 - Up to two ADUs per lotconditionalverify locally
In GMA urban growth areas, at least two ADUs must be allowed where single-family homes are permitted (EHB 1337).
source · reviewed 2026-06-20