Washington State's House Bill 1337 fundamentally changed ADU rights across the state. Passed in 2023 and phased in through 2025, HB 1337 requires cities to allow ADUs on most residential lots, eliminates owner-occupancy requirements, and prevents HOAs from blocking ADU construction. It is the most significant ADU legislation in Washington history.
As of 2026, all Washington cities have been required to update their codes to comply. Some have gone further than the minimum, creating even more ADU-friendly environments. This guide explains what the law requires statewide, how Bellingham and Whatcom County have implemented it, and what rights you have as a property owner.
Understanding these laws is the first step. To find out exactly what you can build on your property, start with our free feasibility study — we apply current local regulations to your specific parcel and give you a clear answer.
HB 1337: Key Provisions
House Bill 1337 was signed into law in 2023. Compliance deadlines were staggered: cities with populations over 25,000 had until June 2025 to update their codes; smaller cities had until December 2025. Here are the law's most impactful provisions:
At Least 2 ADUs Per Lot
Cities over 25,000 (including Bellingham)Cities with populations over 25,000 must allow at least two ADUs on every residential lot. This can be two detached, two attached, or one of each. Cities under 25,000 must allow at least one.
No Owner-Occupancy Requirement
All jurisdictions statewideCities cannot require the property owner to live on the property as a condition of having an ADU. This removes one of the biggest historical barriers to ADU investment.
No Off-Street Parking Requirement (Near Transit)
Properties within 1/2 mile of transitCities cannot require additional off-street parking for ADUs located within half a mile of a major transit stop. Most Bellingham neighborhoods qualify under this provision.
HOAs Cannot Prohibit ADUs
All jurisdictions statewideDeed restrictions, covenants, conditions, and HOA bylaws that prohibit ADUs are unenforceable in areas where the local jurisdiction permits ADUs.
Minimum 1,000 Sqft Must Be Allowed
All jurisdictions statewideCities cannot cap ADU floor area below 1,000 square feet. If a city wants to set a maximum size, it must be at least 1,000 sqft. Many cities allow larger ADUs.
No Minimum Lot Size for ADUs
All jurisdictions statewideCities cannot impose a minimum lot size as a condition for building an ADU. If your lot is zoned residential, you have ADU rights regardless of lot size.
Reduced Setback Requirements
All jurisdictions statewideCities cannot require setbacks for ADUs that are greater than those required for the primary dwelling. Many cities have adopted 5-foot rear and side setbacks for ADUs.
Streamlined Permitting
All jurisdictions statewideCities must process ADU permits administratively (no discretionary review) if the ADU meets objective standards. This eliminates lengthy public hearing processes.
City Compliance Status (2026)
All Washington cities were required to update their ADU regulations to comply with HB 1337. Here's the status of major cities in the Bellingham area and across Washington:
Bellingham
CompliantUpdated December 2025. Allows two ADUs per residential lot. Updated BMC 20.10.036 with reduced setbacks, eliminated owner-occupancy, and streamlined permitting process. Bellingham went beyond the minimum requirements in several areas.
Whatcom County (Unincorporated)
CompliantUpdated WCC 20.80.910 for compliance. County rules differ from Bellingham in several ways, particularly around septic requirements, lot coverage, and the Lake Whatcom Watershed.
Seattle
CompliantSeattle was already more permissive than HB 1337 in several areas prior to the law. Updated to ensure full compliance with all provisions. Allows ADUs in all single-family zones.
Other Washington Cities
Compliance variesMost cities updated their codes by the deadline. Some smaller jurisdictions are still finalizing their updated ordinances. If you're building outside the Bellingham area, check with your local planning department for current status.
What Cities Can Still Regulate
HB 1337 didn't strip cities of all ADU oversight. Cities retain significant regulatory power over how ADUs are built — they just can't use that power to effectively prohibit them. Cities can still regulate:
Design standards (materials, roof pitch, window style) that are objective and predictable
Setback distances (within limits — cannot exceed primary dwelling setbacks)
Utility connection requirements and standards
Critical area protections (wetlands, steep slopes, flood zones)
Building code compliance and inspection requirements
Stormwater management and impervious surface limits
Fire access and emergency vehicle requirements
The key distinction: regulations must be objective (clear, measurable standards) rather than subjective (discretionary aesthetic judgments). A city can say “siding must be wood, fiber cement, or brick” but cannot say “the ADU must be aesthetically compatible with the neighborhood” if that standard is left to interpretation.
What Cities Cannot Restrict
These are the core protections HB 1337 provides to property owners. Cities may not:
Number of ADUs below the minimum (2 for cities over 25,000, 1 for smaller cities)
Owner-occupancy requirements
Off-street parking within 1/2 mile of major transit stops
Total prohibition of ADUs on residential lots
Minimum lot sizes as a condition for ADU construction
Maximum ADU size below 1,000 square feet
Requiring public hearings or discretionary review for code-compliant ADUs
Enforcing HOA or CC&R restrictions that prohibit ADUs
Bellingham's Implementation
Bellingham updated its ADU regulations in December 2025 with amendments to BMC 20.10.036. The city went beyond the minimum HB 1337 requirements in several areas:
What Changed
- Two ADUs allowed per lot (up from one)
- Owner-occupancy requirement eliminated
- Reduced rear and side setbacks to 5 feet
- Off-street parking eliminated near transit
- Administrative (non-discretionary) permitting
Bellingham-Specific Rules
- Design review required in historic districts
- Enhanced stormwater controls in Lake Whatcom Watershed
- Critical area buffers still apply
- Maximum lot coverage still applies per zone
- Separate utility laterals required for detached ADUs
For a detailed comparison of Bellingham and Whatcom County ADU rules, see our Bellingham vs. Whatcom County comparison guide.
Whatcom County's Implementation
Unincorporated Whatcom County has its own ADU regulations under WCC 20.80.910. While the county must comply with HB 1337, there are important differences from Bellingham's implementation:
Key County Differences
- • Septic systems: Properties not connected to municipal sewer must demonstrate adequate septic capacity. A failing or undersized system may need to be upgraded before an ADU can be permitted.
- • Lake Whatcom Watershed: The county administers significant portions of the Lake Whatcom Watershed, where strict impervious surface limits and enhanced stormwater controls apply. ADU construction is possible but more restricted.
- • Well water: Properties on private wells must demonstrate adequate water rights and supply. In some areas, new connections or water rights may be limited.
- • Rural setbacks: Unincorporated areas may have different setback requirements than the city, particularly in agricultural zones.
If your property is in unincorporated Whatcom County, our feasibility study identifies which county-specific regulations apply and how they affect your ADU options.
Impact on Property Owners
HB 1337 dramatically expanded property rights for Washington homeowners. The practical impacts are significant:
ADUs allowed per residential lot in Bellingham
Owner-occupancy requirements (eliminated statewide)
Square feet that must be allowed for ADU floor area
Expanded Income Potential
With two ADUs allowed per lot and no owner-occupancy requirement, a single-family lot in Bellingham can generate income from up to three units: the main house and two ADUs. At current Bellingham rental rates, two ADUs could generate $3,000-$4,400/month in combined rental income.
Increased Property Values
Properties with ADU rights (essentially all residential properties) are worth more because they carry development potential. Properties with permitted, income-producing ADUs are worth significantly more. ADU-friendly regulations increase property values even for owners who never build one.
Investment Flexibility
Eliminating owner-occupancy opens the door for investors, non-resident owners, and landlords to add ADUs. This increases the total number of ADUs that will be built statewide, contributing to housing supply and providing more opportunities for homeowners to build equity through their property.
What's Next for Washington ADU Law
HB 1337 was a landmark step, but ADU policy in Washington continues to evolve. Here's what's on the horizon:
- Pre-approved ADU plans: Several Washington cities are developing pre-approved ADU plan libraries that allow homeowners to skip design and go straight to permitting with a proven floor plan. This can save $10,000-$20,000 in design costs and weeks of permitting time.
- Density bonus programs: Some cities are exploring density bonuses that allow more than two ADUs per lot in exchange for affordable housing commitments or other public benefits.
- Impact fee reductions: Legislation to reduce or eliminate impact fees for ADUs is under discussion, which could reduce project costs by $5,000-$15,000.
- Short-term rental regulations: Cities are still working out how short-term rental rules apply to ADUs. Some permit it, some restrict it, and others are developing new frameworks.
Know Your ADU Rights. Build With Confidence.
Washington law gives you the right to build. Our feasibility study shows you exactly what you can build on your specific property, under current regulations. Free, no obligation, 48-hour turnaround.
Get Your Free Feasibility StudyWe apply current 2026 regulations to your specific parcel.
Frequently Asked Questions
Does HB 1337 override local city ADU regulations?
Yes, in significant ways. HB 1337 sets a floor — minimum standards that all cities must meet. Cities cannot prohibit ADUs on residential lots, cannot limit lots to fewer than two ADUs (in cities over 25,000), cannot require owner-occupancy, and cannot impose excessive setback or parking requirements. However, cities retain the ability to regulate design standards, utility connections, critical area protections, and other aspects that don't conflict with HB 1337's provisions.
Can I build two ADUs on my lot in Washington State?
In cities with populations over 25,000, yes. HB 1337 requires these cities to allow at least two ADUs per residential lot. This includes Bellingham (population ~95,000). The two ADUs can be any combination: two detached, two attached, or one of each. Cities under 25,000 must allow at least one ADU. Some cities may allow more than the minimum required by state law.
Do I need to live on the property to have an ADU in Washington?
No. HB 1337 eliminated owner-occupancy requirements statewide. You can own a property with a main house and one or two ADUs without living in any of the units. This was one of the most significant changes in the law, as previous owner-occupancy rules effectively prevented many investors and non-resident owners from building ADUs.
Can my HOA prevent me from building an ADU?
No. Under HB 1337, HOAs and CC&Rs cannot prohibit ADUs in areas where the local jurisdiction allows them. This provision overrides existing deed restrictions, HOA bylaws, and covenants that attempt to ban accessory dwelling units. While HOAs may still have design review processes, they cannot use those processes to effectively block an ADU that complies with local zoning.
The Law Is on Your Side. Let's Build.
Washington gave you the right to build an ADU. We'll help you exercise it — from feasibility through permitting to move-in.
Check My Property NowCheck Your Property's ADU Potential
Get a free feasibility report based on Whatcom County GIS data and 2025 land use rules.
Get Your Free Feasibility Study
