Washington ADU Laws and Permits: A Comprehensive Resource

Statutory Requirements for ADUs in Washington State

Since HB 1337 became effective in July 2023, state law (RCW 36.70A.680 and 36.70A.681) mandates that all local governments under Washington’s Growth Management Act (GMA) — including all counties (within unincorporated urban growth areas) and cities, regardless of population size — must update their regulations to comply with the following requirements within six months after their periodic review due date:

• Minimum number of ADUs per lot: Two ADUs per lot must be allowed in all GMA urban growth areas, in addition to the principal unit, for lots that meet the minimum lot size required for the principal housing unit. Local regulations must permit ADUs to be attached, detached or a combination of both types. In addition, a conversion of an existing structure, such as a detached garage, must be allowed.
• Maximum ADU size standard: Local governments may not require ADUs to be smaller than 1,000 gross square feet in size.
• Dimensional standards: A local government may not impose setback requirements, yard coverage limits, tree retention mandates, or restrictions on entry door location that are more restrictive than those required for the principal unit.
• Roof height limits: A local government may not impose roof height limits on ADUs that are less than 24 feet unless the principal unit has a lower height limit. (Sec. 4(1)(g))
• Setback requirements: A local government must allow ADUs to be placed at the lot line if the lot line abuts a public alley. (Sec. 4(1)(i))
• Street improvements: A local government may not require street improvements as a condition of permitting accessory dwelling units.
• Owner occupancy: A local government may not require owner occupancy for a principal unit or ADUs.
• Condominium sales: Local governments may not prohibit the sale or other conveyance of a condominium unit independently of a principal unit solely on the grounds that the condominium unit was originally built as an ADU.
• Design review: Local governments may not impose aesthetic standards or requirements for design review that are more restrictive for ADUs than those for principal units.
• Required parking: There are restrictions on how much on-site parking can be required, with a sliding scale for smaller-sized lots. No on-site parking standards may be applied to ADUs located within a half-mile of a major transit stop.
• General parking limits: A local government may not require more than one off-street parking space per unit for lots smaller than 6,000 square feet, or more than two off-street parking spaces per unit for larger lots. (Sec. 4(2)(a)(ii), Sec. 4(2)(a)(iii))
• ADUs as condominiums: A local government may not prohibit ADUs from being sold or conveyed independently of the principal unit solely because they were originally built as ADUs. (Sec. 4(1)(k))
• Impact fees: Impact fees for ADUs are limited to no more than 50% of those assessed to the principal housing unit.
• Common Interest Communities: New “Common Interest Communities” (for example, a new subdivision with a homeowners association) are prohibited from adopting covenants, conditions, and restrictions (commonly called “CC&Rs”) that would limit the construction of ADUs on any lot. Existing CC&Rs, however, are not impacted by the new law and may remain in effect.

Examples of ADU Codes in Washington State

Below are examples of selected ADU codes and ordinances from Washington State that contain code sections that comply with different aspects of HB 1337 as described in the example annotations.

For detailed guidance on compliance with all statutory requirements, including more code examples, see Commerce’s Guidance for Accessory Dwelling Units in Washington State (2023).

Note that local governments planning under the GMA, will revise their regulations as needed to fully conform to the 2023 state requirements within six months after their periodic update due date, as discussed in the above section on statutory requirements. As GMA planning cities and counties update their codes to be fully compliant, we will be posting and noting those examples on this page.

• • Bellingham
    • Ordinance No. 2023-08-022 (2023) – Amends rules to be consistent with HB 1337 before the statutory deadline to do so, to alleviate housing shortages, with the exception of the owner occupancy requirement, which is retained until preempted by law.
    • Municipal Code Sec. 20.10.036 – Consistent with HB 1337 except for the owner occupancy requirement, which is retained until preempted by law.
  • Chelan
    • Municipal Code Sec. 17.20.20(A)(c) – Limits ADUs in its single-family residential district to 1,200 square feet or no more than 50% of the total square footage of the primary residence, whichever is less. The planning director may approve an increased size to efficiently use all floor area if all other standards are met.
  • Enumclaw
    • Municipal Code Sec. 19.34.050 – Allows ADUs on lots of any size.
  • Fircrest
    • Municipal Code Sec. 22.58.012(C)(9) – Doesn’t require additional off-street parking for ADUs unless the planning director determines there is insufficient on-street parking to satisfy parking demand.
  • Kenmore
    • Municipal Code Sec. 18.73.100(E) – Allows detached ADUs up to 35 feet.
  • La Center
    • Municipal Code Ch. 18.247.050(4)(c) – Allows detached ADUs at the rear yard lot line if adjacent to an alley.
  • Lake Stevens
    • Municipal Code Sec. 9.25.010 – Reduces utilities connection fees for ADUs based on ADU size.
  • Seattle
    • Municipal Code Sec. 23.44.041 – Does not require owner occupancy.
  • Sedro Woolley
    • Municipal Code Sec. 17.100.030(K) – Allows the planning director to approve interesting detached ADU designs that are dissimilar from the primary structure.
  • Washougal
    • Municipal Code Sec. 18.46.020(2) – Does not charge an application fee for detached ADU development.

ADU Builders in Washington

Below are accessory dwelling unit contractors in Washington.