Know Before You Build
What does it cost to build an ADU in King County? Get transparent pricing ranges, a breakdown of every cost category, and financing options — so you can plan with confidence.
By the Numbers
King County ADU construction costs vary significantly by type, size, site conditions, and finish level. The table below shows typical ranges for each ADU type — all based on current King County market data including Seattle, Bellevue, Redmond, and surrounding cities.
| ADU Type | Typical Cost Range | Cost per Sq Ft | Best For |
|---|---|---|---|
| Detached ADU (DADU / Backyard Cottage) | $180,000 – $450,000+ | $250 – $450/sq ft | Max rental income, maximum privacy |
| Attached ADU | $120,000 – $280,000 | $200 – $350/sq ft | Leveraging existing structure |
| Garage Conversion ADU | $80,000 – $160,000 | $150 – $280/sq ft | Cost-effective, fastest to permit |
| Basement ADU | $60,000 – $130,000 | $120 – $250/sq ft | Lowest upfront cost, existing footprint |
| Junior ADU (JADU) | $30,000 – $80,000 | $80 – $160/sq ft | Cheapest option, fastest to complete |
Note: Ranges reflect King County market pricing as of 2025–2026. Costs include construction, but not design fees, permits, or utility connections — see Soft Costs below for those. Actual costs vary by site and builder — request a free estimate for your property.
Full Cost Picture
Hard Construction
Foundation, framing, roofing, exterior siding, windows, doors, insulation, drywall, flooring, cabinetry, countertops, interior finishes, plumbing fixtures, electrical panel and wiring, HVAC system.
Soft Costs
Architectural drawings, structural engineering, energy compliance documentation, permit application fees, plan review fees, and inspection fees. Typically 10–18% of hard construction costs.
Site Work
Grading and excavation, utility trenching (water, sewer, gas, electrical), concrete flatwork (path, patio), demolition of existing structures if needed. Highly site-dependent — slopes and difficult access add cost.
Contingency
Always budget a 10–15% contingency on top of your contract price. Unforeseen conditions — soil issues, hidden utilities, permit changes — are common in King County's older housing stock. Don't skip this buffer.
Plan Accordingly
01
Sloped lots require more excavation, retaining walls, and structural engineering. Tight access for equipment adds labor hours. Flat, accessible lots with good yard access are the least expensive to build on.
02
If your property already has available capacity in its water, sewer, gas, and electrical service, connection costs are minimal. Upgrading a panel, adding a service lateral, or extending sewer lines can add $10,000–$40,000.
03
Larger ADUs cost more — but cost per square foot often decreases with size. Vaulted ceilings, high ceilings, and lofts all add material and labor. A compact, efficient floor plan keeps costs lower without sacrificing livability.
04
Standard finishes (LVP flooring, flat-panel cabinets, basic fixtures) keep costs at the lower end. Premium finishes (hardwood, custom cabinetry, high-end appliances) can add $30,000–$80,000+ to any ADU type.
05
Seattle and Bellevue have higher permit fees, longer review timelines, and sometimes design review requirements that add cost. Unincorporated King County and smaller cities often have simpler, faster, cheaper permit processes.
06
A simple rectangular floor plan with a standard roof is far less expensive than a complex multi-level design with a shed dormer or covered porch. Simple architecture also permits faster and reduces plan review cycles.
Beyond Construction
Soft costs — everything beyond hard construction — are often underestimated. Budget these carefully.
Design & Engineering
Design-build firms typically charge $10,000–$25,000 for full design services for a detached ADU. Hiring a separate architect adds $15,000–$40,000. Structural engineering adds $3,000–$8,000. Energy compliance (Washington State Energy Code) is often bundled into design fees.
Permit Fees by City
Permit fees vary significantly: Seattle ($8,000–$20,000+ for a DADU), Bellevue ($5,000–$15,000), Redmond/Kirkland ($4,000–$10,000), Renton/Kent ($3,000–$8,000), Unincorporated King County ($4,000–$10,000). Fees are based on project valuation.
Utility Connections
Water and sewer connection/tap fees range from $5,000–$20,000 depending on your utility district. A new electrical service or panel upgrade runs $3,000–$10,000. Some older properties need gas line extensions ($2,000–$8,000).
Other Soft Costs
A boundary survey costs $1,500–$4,000 and may be required to confirm setbacks. Some sites require a geotechnical (soils) report ($3,000–$8,000). Impact fees in some cities add $5,000–$15,000 for a new dwelling unit.
Fund Your Project
Most Popular
If you have equity in your King County home, a Home Equity Line of Credit (HELOC) or Home Equity Loan is typically the lowest-cost financing option. Rates are typically 1–2% lower than construction loans. Requires significant existing equity (usually 20%+ after the draw).
Flexible Access
Refinancing your existing mortgage to pull out equity can fund an ADU build if current mortgage rates are favorable. This replaces your primary mortgage, so it's best evaluated carefully alongside HELOC options depending on your current rate and loan balance.
Construction Phase
A standalone construction loan funds the build in draws as milestones are completed, then converts to a permanent mortgage. More complex to set up but appropriate for larger ADU projects where existing equity is limited. Requires strong credit and detailed project plans.
All-in-One
The HomeStyle loan wraps purchase and renovation (or ADU construction) into a single mortgage. Useful if you're buying a property with the intent to add an ADU. Requires a licensed contractor and approved construction plans before closing.
State Programs
Washington State and some King County cities offer ADU-specific loan programs, permit fee waivers, and expedited review for units committed to affordable rental rates. The Washington State Housing Finance Commission has resources worth exploring before you finance.
No Debt Option
For garage conversions, basement ADUs, or JADUs — where costs range from $30,000–$130,000 — many King County homeowners fund construction directly from savings. No interest costs, no lender requirements, and faster project launch.
The Business Case
An ADU is one of the most financially productive additions you can make to a King County property. Here's how the numbers typically work out.
$1,800
A well-located 400–600 sq ft ADU in King County rents for $1,500–$2,100/month depending on city, amenities, and finish level. Higher in Seattle proper and on the Eastside.
$2,400
A larger detached ADU (700–1,000 sq ft) with a bedroom, dedicated parking, and backyard access typically commands $2,100–$2,800/month in King County's competitive rental market.
8–11%
A $220,000 garage conversion generating $1,800/month in rent produces a ~9.8% annual gross yield. After expenses (insurance, maintenance, vacancy), net yield is typically 6–8% — well above typical equity returns.
$200K+
King County ADUs typically add $150,000–$350,000 to a home's assessed market value, depending on ADU type, size, and location. This uplift often represents 1.2x–1.5x the cost of construction — immediate equity gain.
Example is illustrative. Actual results vary by location, property condition, finish level, and rental market conditions.