One of the most impactful provisions of HB 1337 is the requirement that all lots in urban growth areas allowing single-family homes must permit at least two ADUs. This effectively triples the potential housing units on a single residential lot.
The Rule
Primary Home
1 dwelling
ADU #1
Attached or detached
ADU #2
Attached or detached
Up to 3 dwelling units on a single residential lot — with ADUs exempt from density limitations.
Source: RCW 36.70A.680, BMC 20.10.036(B)(3)(a)
Allowed Configurations
You can combine ADU types in any way that works for your property:
2 Detached ADUs
Two separate backyard cottages. Best for large lots with enough space for setbacks and access paths. Maximum development potential.
Best for: Large lots (10,000+ sq ft) like Birchwood
Example: Primary home + 800 sq ft cottage + 600 sq ft cottage
2 Attached ADUs
Two ADUs within or attached to the primary structure. Could be a basement unit + an above-garage unit, or two separate additions.
Best for: Smaller lots where detached construction is tight
Example: Primary home + basement ADU + garage-top ADU
1 Attached + 1 Detached
The most common "mixed" approach. Convert an existing space (basement, garage) for one ADU and build new for the second.
Best for: Most residential lots — flexible and cost-effective
Example: Primary home + basement conversion + new backyard cottage
Garage Conversion + New Build
Convert your existing garage to ADU #1 (often the cheapest option), then build a new detached ADU for #2.
Best for: Properties with existing detached garages
Example: Primary home + converted garage ADU + new 800 sq ft cottage
Size Limits
| Jurisdiction | Max per ADU | Notes |
|---|---|---|
| Bellingham | 1,000 sq ft | CUP available for larger detached ADUs. Basement conversions in homes 5+ years old are exempt from size limit. |
| Whatcom County (UGA) | 1,248 sq ft | Must be subordinate to primary residence. |
| State minimum | 1,000 sq ft | Cities cannot set maximum below this. This is a floor, not a ceiling. |
Practical Considerations
Setbacks still apply
Each ADU must meet setback requirements. On smaller lots, this can limit where (and how large) a detached ADU can be. Consider an attached ADU to maximize space.
Utility capacity matters
Two ADUs means three total units drawing water, sewer, and electrical. Ensure your service connections can handle the load — upgrades may be needed.
Parking may or may not be required
Near transit (1/2 mile), no additional parking is required. Otherwise, check your specific zone requirements. Eliminating parking requirements saves space and money.
Build in phases
You don't have to build both ADUs at once. Many homeowners start with one (often a conversion), generate rental income, then build the second.
One primary dwelling max
The lot may contain only one primary dwelling unit. Both ADUs are accessory to it. You cannot build two full-size houses and call one an ADU.
The Investment Math
A typical Bellingham example with two ADUs:
Estimated rental rates for Bellingham as of 2026. Actual rents vary by neighborhood, size, finish level, and market conditions. Owner-occupancy no longer required — all three units can be rented.
See our full ADU cost breakdown to compare construction costs against potential income. Need help with permitting for two ADUs? Our permit specialists handle both applications.
Last updated: March 2026. Not legal advice. Verify current regulations with your local planning department.
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