House Bill 1337, signed into law on April 6, 2023 and effective July 5, 2023, fundamentally changed how Washington cities and counties must treat Accessory Dwelling Units. The law establishes statewide minimum standards that local jurisdictions cannot restrict below — effectively creating a "right to build" for ADUs across the state.
Key dates: Whatcom County compliance deadline was June 30, 2025. Bellingham adopted its compliance ordinance on December 8, 2025. If a jurisdiction doesn't comply, state standards under RCW 36.70A.535 automatically take effect.
What HB 1337 Requires
Two ADUs Per Lot
All lots in urban growth areas that allow single-family homes must permit at least two ADUs. Any configuration is allowed: 2 attached, 2 detached, or 1 of each. ADUs are exempt from density limitations.
What this means: A Bellingham homeowner can now build a backyard cottage AND convert their basement — both as permitted ADUs on a single lot. See our guide to two ADUs per lot for configuration options.
Source: RCW 36.70A.680
Owner Occupancy Eliminated
Cities and counties may NOT require the owner to live in the ADU or the primary residence. No more annual affidavits. Homeowners can rent both the main home and ADU(s) independently. Effective July 1, 2025 statewide.
What this means: Investors can purchase a property, build an ADU, and rent both units — without ever living on-site. Read our owner occupancy guide for the full timeline.
Source: RCW 36.70A.681(1)(b)
1,000 Sq Ft Minimum Floor
Local governments cannot set maximum ADU size below 1,000 sq ft. This is a floor — cities can allow larger, but cannot restrict below this threshold.
What this means: Previously, some jurisdictions capped ADUs at 800 sq ft or a percentage of the main home. That's no longer allowed.
Source: RCW 36.70A.681(1)(c)
Impact Fee Limits
Impact fees for ADU construction cannot exceed 50% of the fees for the principal unit. Whatcom County currently does not assess impact fees for ADUs.
What this means: Lower upfront costs make ADU projects more financially accessible. See our full ADU cost breakdown for current Whatcom County numbers.
Source: RCW 36.70A.681(1)(a)
Parking Relaxation
No parking required if the ADU is within 1/2 mile of a major transit stop. This applies to many Bellingham neighborhoods near WTA routes.
What this means: Eliminating a parking space saves $5,000-$15,000 in construction costs and frees up buildable area on your lot.
Source: RCW 36.70A.681(1)(e)
Existing Structure Conversion
Jurisdictions must allow conversion of existing legal nonconforming structures into ADUs — even if the structure doesn't meet current setback or lot coverage standards.
What this means: That old garage that's 3 feet from the property line? You can convert it to an ADU without having to tear it down and rebuild to current setbacks. See our garage conversion services.
Source: RCW 36.70A.681(1)(d)
Condo Ownership
ADUs may be sold as condominiums independent of the principal unit. The land stays with the primary homeowner, but the ADU structure can have separate ownership.
What this means: This creates a new asset class — individual ADU units that can be bought and sold independently.
Source: RCW 36.70A.681(1)(g)
What HB 1337 Does NOT Do
Does NOT override short-term rental restrictions
Individual cities can still ban or regulate Airbnb/VRBO for ADUs. HB 1337 specifically does not prohibit local STR restrictions.
Does NOT override environmental protections
Critical areas, watershed protections, and flood zone regulations still apply. The Lake Whatcom Watershed ban on ADUs remains in effect.
Does NOT eliminate building code requirements
ADUs still must meet all building, plumbing, electrical, and energy code requirements. Construction permits are still required.
Does NOT override HOA restrictions (directly)
While state law generally preempts HOA restrictions that conflict with ADU law, enforcement is evolving. Communities like Sudden Valley may still attempt to restrict ADUs through CC&Rs.
Ready to take advantage of HB 1337? Our permit specialists handle the entire process for a $1,500 deposit — applied to your project cost. Or start with a full cost breakdown to understand the investment.
How Whatcom County Implemented HB 1337
City of Bellingham
Adopted compliance ordinance December 8, 2025. Updated BMC 20.10.036 to allow two ADUs per lot, streamline Type I permitting, and align with state mandates.
Full Bellingham code guideWhatcom County
Updated 2025 Comprehensive Plan with UGA amendments. WCC 20.80.910 now allows two ADUs in UGAs, condo ownership, zero-lot-line construction, and existing structure conversions.
Full county code guideOfficial Sources
- WA RCW 36.70A.680-681: app.leg.wa.gov
- City of Bellingham ADU page: cob.org
- Whatcom County ADU page: whatcomcounty.us
Last updated: March 2026. Not legal advice. Verify current regulations with your local planning department.
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