Real all-in pricing — no line items left out.
Detached ADUs in King County start at $400,000 all-in — that means design, permits ($20K–$50K), site prep, utility connections ($15K–$40K), and construction. If you've seen quotes in the $250K–$350K range, ask the contractor what's excluded. Permits and site work alone routinely add $40,000–$90,000. At Seattle rents of $2,400–$3,200/month the ROI is compelling — but only with accurate numbers upfront. Get our free feasibility study to see exactly what an ADU costs on your specific property.
How much does it cost to build an ADU in Seattle? ADU building costs in King County range from $200,000 for a garage conversion to $800,000+ 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
$200,000 – $325,000+
- 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
$225,000 – $400,000+
- Use existing space below grade
- Shared utility connections
- No additional lot coverage in most cases
Best for: Homeowners with existing basements and adequate ceiling height (7 ft minimum).
Attached ADU
$300,000 – $500,000+
- 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)
$400,000 – $800,000+
- All-in: design, permits, site prep, utilities, and construction
- Separate structure — maximum privacy and rental income
- Strongest property value add of any ADU type
Best for: Homeowners with adequate lot size who want maximum rental income or resale value. Note: quotes in the $250K–$350K range typically exclude permits ($20K–$50K), site prep, and utility connections — ask what's included.
Additional Costs to Budget For
| Item | Estimated Range |
|---|---|
| Seattle permit fees & impact fees | $20,000 – $50,000 |
| Utility connections (SPU water/sewer, Seattle City Light) | $15,000 – $40,000 |
| Site preparation & grading | $5,000 – $25,000 |
| Septic upgrade (King County rural only) | $15,000 – $30,000 |
| Shoreline permit (SMA areas only) | $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 King County ADU cost breakdown.
Rural Properties: Budget for Septic
In unincorporated King 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 Seattle 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.
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.
