Washington State ADU Permit Requirements by Jurisdiction

Last Updated: March 2026

Accessory Dwelling Units (ADUs) have become one of the fastest-growing housing types in Washington State. In 2023, over 1,800 ADU permits were issued in King County alone. This guide covers current permitting requirements across Washington jurisdictions.

Statewide ADU Legislation

HB 1337 (2023) - The ADU Reform Act

Washington's landmark ADU legislation requires all cities to allow:

  • At least two ADUs per lot in residential zones
  • One attached ADU (converted garage, basement, etc.)
  • One detached ADU (backyard cottage, DADU)
  • Cities cannot require owner-occupancy for lots with ADUs
  • Parking requirements eliminated for ADUs near transit

Maximum Size Limits Under State Law

Per HB 1337, cities cannot restrict ADUs to less than:

  • 1,000 sq ft for detached ADUs
  • 1,000 sq ft for attached ADUs
  • The larger of 1,000 sq ft OR 60% of principal dwelling

Seattle ADU Requirements

Current Seattle ADU Rules (2024+)

Seattle allows the most liberal ADU development in Washington:

Permitted Unit Types:

  • One attached ADU (AADU)
  • One detached ADU (DADU)
  • Both on the same lot simultaneously

Size Limits:

  • AADU: No maximum (limited by lot coverage)
  • DADU: 1,000 sq ft (or 12% of lot area in some zones)

Permit Process:

Permit Type Timeline Cost (2026)
Subject-to-Field-Inspection (STFI) 2-4 weeks ~$2,500
Full Building Permit 8-16 weeks ~$5,000-15,000
Design Review (if required) Add 3-6 months ~$4,500

Seattle-Specific Requirements:

  • No off-street parking required
  • No owner-occupancy requirement
  • Must connect to city sewer (no septic)
  • Separate utility meters required

Seattle Pre-Approved ADU Plans

Seattle offers pre-approved ADU plans that streamline permitting:

King County (Unincorporated Areas)

Requirements for Unincorporated King County

Permitted Units:

  • One ADU per lot in RA (Rural Area) zones
  • Must be on lot with existing single-family residence

Size Limits:

  • 1,500 sq ft maximum or 60% of primary dwelling
  • Height limit: 24 feet (one story preferred)

Key Requirements:

  • Owner-occupancy required in one of the units
  • Must meet Rural Character Guidelines
  • Septic capacity verification required
  • Fire sprinklers may be required if remote

Permit Costs (2026):

  • Building permit: $3,000-8,000
  • SEPA review (if required): $1,500
  • Impact fees: Varies by location

Bellevue ADU Requirements

Current Bellevue Rules:

  • One ADU permitted per lot
  • Maximum 1,000 sq ft or 40% of primary dwelling
  • Must be attached or within 10 feet of primary dwelling
  • No owner-occupancy requirement (updated 2024)

Bellevue Permit Timeline:

  • Building permit: 6-10 weeks
  • Permit fee: ~$4,000-8,000

Tacoma ADU Requirements

Tacoma's Progressive ADU Policy:

  • Two ADUs permitted per lot (attached + detached)
  • Maximum 1,000 sq ft for detached
  • No owner-occupancy requirement
  • Impact fee exemption for affordable ADUs

Permit Process:

  • Standard permit: 4-8 weeks
  • Pre-approved plans available (faster review)

Spokane ADU Requirements

Spokane Rules (Updated 2024):

  • One ADU per lot in residential zones
  • Maximum 1,200 sq ft or 75% of primary dwelling
  • Height limit: 25 feet
  • Must match primary dwelling aesthetics

Spokane Permit Timeline:

  • 4-6 weeks for standard ADU
  • Permit fees: ~$2,500-5,000

Bellingham ADU Requirements

Current Rules:

  • One ADU per lot in most residential zones
  • Maximum 800 sq ft (smaller than state maximum)
  • Height limit: 18 feet
  • Side setback: 5 feet minimum

Notable Bellingham Requirements:

  • Deed restriction required (recorded covenant)
  • Design standards for visibility from street
  • Parking: 1 space required (unlike Seattle)

Vancouver (Clark County) ADU Requirements

Vancouver Rules:

  • One ADU per lot
  • Maximum 900 sq ft or 40% of lot area
  • Must connect to city sewer
  • Owner-occupancy required in one unit

Olympia ADU Requirements

Current Rules:

  • Two ADUs permitted per lot
  • Maximum 1,200 sq ft
  • No owner-occupancy requirement
  • Impact fee reduction available

Contractor Licensing for ADU Construction

Required Licenses

ADU construction typically requires:

  • General Contractor License per RCW 18.27
  • Electrical Contractor License for wiring (RCW 19.28)
  • Plumbing Contractor License for plumbing (RCW 18.106)

Finding Licensed ADU Contractors

Verify contractor credentials at:

Common ADU Permit Challenges

Issues That Cause Delays

  1. Septic capacity - Rural areas may need new/expanded septic
  2. Utility connections - Sewer/water tap fees can exceed $20,000
  3. Tree removal - Many jurisdictions require tree retention plans
  4. Stormwater - May trigger drainage requirements
  5. Design review - Some neighborhoods require architectural review

How to Speed Up Permitting

  • Use pre-approved plans (Seattle, Tacoma)
  • Hire an experienced ADU designer/architect
  • Complete pre-application meeting with city
  • Submit complete permit application (avoid corrections)
  • Consider modular/prefab ADUs with type-approval

ADU Costs in Washington

Typical ADU Construction Costs (2026)

ADU Type Cost Range Cost/sq ft
Garage conversion $80,000-150,000 $150-250
Basement conversion $100,000-200,000 $175-300
Detached new construction $200,000-400,000 $300-500
Prefab/modular ADU $150,000-300,000 $275-400

Additional Costs

  • Permits & fees: $5,000-25,000
  • Utility connections: $5,000-30,000
  • Site work: $10,000-50,000
  • Design/architecture: $5,000-20,000

Related Resources

ADU regulations vary significantly by jurisdiction. Always verify current requirements with your local planning department before beginning design or construction.