Accessory Dwelling Unit (ADU) Rules: State of Washington and Bellingham Highlights

Washington State is making it easier than ever to build an ADU or backyard cottage. Starting July 1, 2025, new state law (HB 1337) unlocks more options for homeowners everywhere—including Bellingham.

Statewide Changes (Effective July 1, 2025):

  • Two ADUs per lot: Homeowners may build both an attached and detached ADU, or two of one type.

  • No owner-occupancy requirement: Except for certain sensitive environmental zones, you won't need to live onsite to build or rent out ADUs.

  • ADU size: Cities must allow ADUs up to 1,000 sq ft at minimum.

  • Parking: No off-street parking required for ADUs within 0.5 miles of major transit routes.

  • Easier permits: Permitting must be faster and less costly statewide, with some communities offering waived or reduced fees for early applications.

Bellingham ADU & DADU Rules (2025):

  • Sizes:

    • Attached ADU: Up to 1,000 sq ft or 60% of main house size (whichever is less).

    • Detached ADU (DADU): Up to 1,000 sq ft, up to 24 ft tall.

    • Basement ADUs: No size limit within existing structure.

  • Setbacks:

    • 5 ft from side/rear, 10 ft between DADU and house.

  • Parking:

    • No parking required within 0.5 miles of frequent transit; otherwise, one off-street space per ADU.

  • Owner-Occupancy:

    • Required only until July 1, 2026 (unless in a special regulated zone, such as the Lake Whatcom Watershed).

  • Short-Term Rentals (STR):

    • Allowed with license/inspection, but stricter rules if detached or in single-family zones. STR hosts generally must reside onsite and are subject to additional caps.

  • Design:

    • New ADUs must use exterior materials compatible with the main home.

  • Garage and Basement Conversions:

    • Permitted with proper permits; garage conversions may use flexible setbacks.

  • Streamlined Permitting:

    • Approval times are dropping to as little as 4–6 weeks. Early applications may qualify for additional fee waivers or expedited review.

Important Notes:

  • Special restrictions apply for building in the Lake Whatcom Watershed and historic districts—check local requirements first.

  • Utility upgrades (like sewer) may be needed in some neighborhoods before permitting.

  • Always get a permit before converting existing space or adding a new ADU to avoid fines and compliance issues.

  • Building an ADU can increase property value, create new housing for family or rental income, and contribute to Bellingham’s long-term sustainability goals.