How much does it cost to build an ADU in Bellingham? It depends on what you are building. Our Whatcom County work runs from $20,000 for an unpermitted insulated studio to $500,000 for a large custom detached ADU. There are three cost tiers, and which tier you are in matters more than square footage.
Want fixed-price certainty instead? Our prefab ADU collection lists 14 factory-built floor plans from 200 to 900 sq ft with published unit prices starting at $109,500, delivered and installed in Whatcom, Skagit, and Island counties.
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.
Unpermitted Accessory Structure
~$200/sq ft
An insulated, enclosed shed, studio, office, or bonus room. Electricity only — no plumbing, no water, not a dwelling. Does not require a building permit.
Permitted Accessory Structure
~$300/sq ft
Permitted and built on a real foundation, used as a bedroom, office, or studio. Still no kitchen and no plumbing.
Full ADU / DADU
$450 – $650/sq ft
A permitted dwelling unit with a kitchen, bathroom, and full utility connections. Broken down by ADU type below; garage and basement conversions reuse an existing shell and run $200 – $300/sq ft.
No minimum project size.
A 120 sq ft backyard office is real work to us, the same as a 1,000 sq ft detached unit. Small jobs are not exceptions — we show up, quote them, and build them.
Full ADU Costs by Type
Detached ADUs below are priced at $450 – $650 per square foot, all-in. An attached ADU runs $350 – $500 per square foot: it is still new square footage, but it shares a wall, part of the roofline and utility runs with the main house. Garage and basement conversions run $200 – $300 per square foot, because the shell, slab and roof already exist: about $200 for an unpermitted, power-only conversion up to about $300 permitted. Conversions are cheaper per foot and usually smaller.
Garage Conversion
$80,000 – $180,000
- Lowest total cost entry point
- Existing structure reduces scope
- Foundation and shell already exist
Best for: Homeowners with a 2-car garage who want to minimize total project cost.
Basement ADU
$80,000 – $210,000
- 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
$175,000 – $400,000
- 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)
$225,000 – $650,000
- 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. We raise this in your free 30-minute consultation so you can budget for it early.
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.
WA HEAR Rebate — Up to $1,750
Washington's Home Electrification and Appliance Rebates (HEAR) program offers up to $1,750 toward qualifying electrification equipment; eligibility rules apply (as of 2026-10-05). The federal 25C energy-efficiency tax credit ended 2025-12-31. Our sustainable builds start from the 2021 WSEC all-electric baseline.
How Our Pricing Works
We believe in transparent pricing with no surprises. Every ADU project starts with a free 30-minute consultation so you know what to expect before spending a dollar.
Free 30-Minute Consultation
No cost, no obligation. A real conversation about your property's zoning, lot constraints, utility access, and a realistic cost range. Site visits and permitting are paid work. Book your free 30-minute consultation here.
Full Permit Package
We handle your complete permitting process — land use and building permits. Your investment converts to a build deposit when you sign the Letter of Intent. Learn more about what the permit package includes.
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.
