An Accessory Dwelling Unit (ADU) is a self-contained residential unit on the same lot as a primary home. Washington State's HB 1337 — combined with Bellingham's December 2025 ordinance update and Whatcom County's 2025 comprehensive plan amendments — has created the most favorable ADU-building environment in the region's history. Most residential lots in urban growth areas can now support up to two ADUs with no owner-occupancy requirement.
This guide covers everything Whatcom County homeowners need to know: what types of ADUs exist, where you can build, what the rules are, how much it costs, and how to get started.
Types of ADUs in Whatcom County
Detached ADU (DADU)
A separate structure on your lot — backyard cottage, guest house, or standalone rental. Maximum privacy and rental value. Up to 1,000 sq ft standard (larger with CUP in Bellingham).
Attached ADU
A unit built within or connected to your existing home — basement conversions, side additions, above-garage units. Lower infrastructure costs since you share structural elements.
Garage Conversion
Convert an existing garage into a permitted dwelling unit. The lowest-cost entry point. Existing structure conversions are allowed even if the building doesn't meet current setback standards.
Basement ADU
Turn existing below-grade space into a rental-ready unit. In Bellingham, basement conversions are exempt from the 1,000 sq ft size limit if the home has been on the property 5+ years.
Key Rules You Need to Know
Two ADUs Per Lot
Most residential lots in urban growth areas can support up to two ADUs in any configuration — two attached, two detached, or one of each. ADUs are exempt from density limitations.
Source: RCW 36.70A.680 / BMC 20.10.036
No Owner Occupancy
You are no longer required to live on the property. You can rent both your main home and your ADU(s) independently. This was eliminated statewide under HB 1337.
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. Cities can allow larger, but cannot restrict below this threshold.
Source: RCW 36.70A.681(1)(c)
Reduced Parking
No parking required if the ADU is within ½ mile of a major transit stop. Many Bellingham neighborhoods qualify.
Source: RCW 36.70A.681(1)(e)
Existing Structure Conversion
You can convert existing legal nonconforming structures (garages, barns) into ADUs even if they don't meet current setback or lot coverage standards.
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.
Source: RCW 36.70A.681(1)(g)
Lake Whatcom Watershed Exception
Properties within the Lake Whatcom Watershed are prohibited from ADU construction under Chapter 16.80 BMC. The watershed serves as Bellingham's primary drinking water source. This is the single biggest exception to ADU-friendly rules in Whatcom County.
Read our Lake Whatcom Watershed guideADU Guides by City
ADU rules vary by jurisdiction. Click your city for a detailed breakdown of local regulations, permit processes, and what to expect.
Bellingham
City rules, BMC 20.10.036, December 2025 ordinance updates, and neighborhood-level details.
Read guideUnincorporated Whatcom County
UGA rules, WCC 20.80.910, rural lot requirements, and 2025 comprehensive plan changes.
Read guideFerndale
Pre-approved ADU designs, dropped CUP requirements, and updated design flexibility.
Read guideLynden
Two-bedroom ADUs now allowed, attached-only in single-family zones, and 4.5+ acre detached requirements.
Read guideWhat Does an ADU Cost?
| ADU Type | Cost Range |
|---|---|
| Basement Conversion | $100K – $200K |
| Garage Conversion | $120K – $200K |
| Attached ADU | $150K – $350K |
| Detached ADU (DADU) | $200K – $450K+ |
Additional costs include permits ($5K\u2013$15K), utility connections ($10K\u2013$30K), and site prep ($5K\u2013$20K). See full ADU cost breakdown for Whatcom County.
Ready to Get Started?
Get a free feasibility study to confirm your lot qualifies, or explore our ADU permitting service — $1,500 deposit, applied to your project cost.
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Last updated: March 2026. This guide is for educational purposes only and does not constitute legal advice.
