How much does it cost to build an ADU in Bellingham? ADU building costs in Whatcom County range from $100,000 for a basement conversion to $450,000+ for a custom detached unit, depending on the type, size, and finish level. Below are our pricing tiers based on real project data from local builds.
For a deeper dive into cost factors by square footage and finish tier, see our complete ADU cost breakdown. For permit fees and pre-construction budgeting, check our permit cost guide.
Garage Conversion
$120,000 – $200,000
- Lowest cost entry point
- Existing structure reduces costs
- Foundation and shell already exist
Best for: Homeowners with a 2-car garage who want to minimize construction costs.
Basement ADU
$100,000 – $200,000
- Use existing space below grade
- Shared utility connections
- Size limit exemption for homes 5+ years old
Best for: Homeowners with existing basements and homes that have been on the property 5+ years.
Attached ADU
$150,000 – $350,000
- Shares structural elements with main home
- Lower infrastructure costs
- Multiple configuration options
Best for: Homeowners who want to add a unit without building a separate structure.
Detached ADU (DADU)
$200,000 – $450,000+
- Maximum privacy and rental value
- Separate structure on your lot
- Highest ROI for most properties
Best for: Homeowners with adequate lot size who want maximum rental income or resale value.
Additional Costs to Budget For
| Item | Estimated Range |
|---|---|
| Permit fees & impact fees | $5,000 – $15,000 |
| Utility connections (sewer, water, electrical) | $10,000 – $30,000 |
| Site preparation & grading | $5,000 – $20,000 |
| Septic upgrade (rural properties) | $15,000 – $30,000 |
| Extended utility runs (deep lots) | $5,000 – $15,000 |
| Critical areas assessment (if required) | $3,000 – $10,000 |
| Architectural design | Included with our build |
Need the full permit fee schedule? Our permit cost guide breaks down every government fee, utility charge, and professional service cost. For size-by-size construction estimates, see the Whatcom County ADU cost breakdown.
Rural Properties: Budget for Septic
In unincorporated Whatcom County, septic system capacity is the single biggest cost variable. Properties on septic may need a system upgrade costing $15,000-$30,000 before an ADU can be permitted. Our free feasibility study identifies this early so you can budget accurately.
Financing Options
HELOC (Home Equity Line of Credit)
The most common ADU financing method. Borrow against your home equity at competitive rates. Many Bellingham homeowners have significant equity to leverage.
Construction Loan
Short-term financing specifically for construction. Funds are disbursed in stages as work progresses. Converts to a standard mortgage upon completion.
ADU-Specific Loan Products
Several lenders now offer loan products designed specifically for ADU construction. These may factor in projected rental income for qualification.
30% Federal Tax Credit
Energy-efficient ADU construction may qualify for the 30% federal energy tax credit. Our sustainable builds are designed to meet efficiency standards that maximize this benefit.
How Our Pricing Works
We believe in transparent pricing with no surprises. Every ADU project starts with a free property analysis so you know what to expect before spending a dollar.
Free Feasibility Study
No cost, no obligation. We analyze your property's zoning, lot constraints, utility access, and give you a realistic cost range. Start your free feasibility study here.
$1,500 Permit Feasibility Deposit
Covers all permit research and analysis. Applied toward your total project cost if you proceed to build. Learn more about what permit costs include.
Construction Proposal
Detailed line-item pricing for your specific project. No guesswork -- you know exactly what you're paying for. See our full 3-step process.
