Overview
An Attached Accessory Dwelling Unit (A-ADU) is a self-contained living space built within or physically connected to your existing home. This includes basement conversions, additions to the side or rear of your home, and second-story additions above a garage.
Attached ADUs are often the most cost-effective option because they share structural elements, utility connections, and sometimes HVAC systems with the primary residence. For homeowners on smaller lots where setbacks limit detached construction, an A-ADU may be the best path forward.
Under Bellingham's updated code (BMC 20.10.036), basement A-ADU conversions are exempt from the 1,000 sq ft size limit if the primary dwelling has been on the property for 5 or more years — a significant advantage for older Bellingham homes with generous basements.
What's Included
- Basement, addition, or above-garage conversion options
- Shared utility connections reduce infrastructure costs
- Separate entrance with direct street access required
- Full kitchen, bathroom, and living space
- Sound insulation between units
- Fire separation per building code
- Permitting handled by our specialist
Need more space? A detached ADU offers maximum privacy and rental value on larger lots. Need help navigating permits? Our permit team handles everything for $1,500.
See the full cost breakdown for Whatcom County or visit our pricing page.
Frequently Asked Questions
How much does an attached ADU cost in Bellingham?
Attached ADUs typically range from $150,000 to $350,000 depending on whether you're converting existing space (cheaper) or building an addition (more expensive). Basement conversions tend to be on the lower end since the shell already exists.
Does my basement ADU count toward the 1,000 sq ft limit?
If your primary home has been on the property for 5+ years, basement A-ADU conversions are exempt from the 1,000 sq ft size limit under BMC 20.10.036(B)(6). This is a significant advantage for older Bellingham homes.
Can I still build a detached ADU if I have an attached ADU?
Yes. Under HB 1337 and Bellingham's ordinance, you can have up to two ADUs per lot in any configuration — two attached, two detached, or one of each. This means a basement conversion plus a backyard cottage is fully permitted.
What are the sound requirements between units?
Building code requires STC (Sound Transmission Class) ratings of 50 or higher between dwelling units. This means proper insulation, double drywall, and resilient channel installations in shared walls and floors.
