Overview
A Detached Accessory Dwelling Unit (DADU) is a self-contained living space built as a separate structure on your property. In Whatcom County, DADUs are the most popular ADU type for homeowners looking to maximize rental income, create multigenerational living arrangements, or build long-term property value.
Under Washington's HB 1337 and Bellingham's December 2025 ordinance update, most residential lots in the city can now support up to two ADUs — meaning you could potentially build a DADU in your backyard alongside a basement conversion in your main home.
Our team handles every phase: site analysis, architectural design, permitting, and construction. Loren runs your project start to finish — feasibility, permitting through Bellingham or Whatcom County planning, and the build itself — alongside Carlos, who runs Monarca Construction, a licensed WA State general contractor, and builds ADUs from framing through finish. Our architect designs to your specific lot constraints and zoning requirements, and on custom builds Jorge, our project designer, leads design and permitting.
What's Included
- Custom architectural design optimized for your lot and setbacks
- Full permitting through Bellingham or Whatcom County
- Foundation, framing, plumbing, electrical, and finish work
- Kitchen, bathroom, and laundry hookups included
- Energy-efficient construction (qualifies for 30% federal tax credit)
- Separate utility connections where required
- Final inspection coordination and certificate of occupancy
Explore your options: Not sure if detached is right for you? Read our DADU vs ADU comparison or compare with an attached ADU for potentially lower costs. Also see backyard cottage options in Bellingham.
See the full cost breakdown for Whatcom County, our cost per square foot guide, or visit our pricing page.
Planning your project? Review the construction timeline, explore financing options, and calculate your potential rental income.
Popular neighborhoods for DADUs: Birchwood, Sunnyland, Cordata, and Columbia.
Frequently Asked Questions
How much does a detached ADU cost in Bellingham?
Detached ADU construction in Whatcom County typically ranges from $225,000 to $500,000 for a 500–1,000 sq ft unit, which works out to $450–$650 per square foot all-in (design, permitting, construction, and utility connections). Finishes, site complexity, and utility runs move you within that range. Our feasibility study includes a preliminary cost estimate specific to your property.
What is the maximum size for a detached ADU in Bellingham?
Under BMC 20.10.036, the standard maximum is 1,000 sq ft. You can exceed this with a Conditional Use Permit approved by the Hearing Examiner. The state minimum floor of 1,000 sq ft (per HB 1337) means Bellingham cannot restrict below this. Floor area includes all attached ancillary space like a garage or workshop.
What are the setback requirements for a DADU?
DADUs must comply with all zoning setback requirements for the primary dwelling. However, a D-ADU may be placed less than 5 feet from the property line if you have a joint agreement with the adjoining property owner or if the planning director determines impacts are negligible.
Do I need a separate driveway for my detached ADU?
No. The ADU and primary residence must share one access (driveway). However, the main entrance to the ADU must have direct access to a street via a lighted pedestrian path, driveway, or alley.
Can I build a detached ADU in the Lake Whatcom Watershed?
No. Properties within the Lake Whatcom Watershed are currently prohibited from ADU construction under Chapter 16.80 BMC. This is to protect Bellingham's primary drinking water source. Our feasibility study checks watershed status automatically.
