Honest All-In Pricing: DADUs Start at $400,000
Detached ADUs (backyard cottages) in Seattle and King County start at $400,000 all-in — design, permits ($20K–$50K), site preparation, utility connections ($15K–$40K), and construction. Quotes in the $250K–$350K range almost always exclude one or more of those categories. Ask any contractor: "Does that include permits, site prep, and utility connections?" If the answer is no, add $40,000–$90,000+ to their number. Our pricing includes everything. Garage and basement conversions offer a lower entry point at $200,000–$325,000 by using an existing structure. Our free feasibility study gives you an accurate, all-in cost estimate for your specific property.
How Much Does an ADU Cost in King County?
The total cost to build an ADU in Seattle and King County ranges from $200,000 to $800,000+ depending on the type, size, site conditions, and finish level. Here's what each type typically costs in 2026:
Garage Conversion
$200,000 – $325,000
$400 – $550/sq ft
Converting an existing garage. Existing structure helps reduce some costs, but full systems (plumbing, electrical, HVAC, finishes) still add up.
Basement ADU
$225,000 – $400,000
$400 – $550/sq ft
Converting an existing basement. Egress windows, plumbing rough-in, electrical panel work, and finishes make this a significant project.
Attached ADU (Addition)
$300,000 – $500,000+
$450 – $650/sq ft
Addition to your existing home. Shares a wall but requires new foundation, roofing, and full systems connection.
Detached ADU (DADU)
$400,000 – $800,000+
$500 – $700/sq ft
New construction from the ground up. All-in price includes design, permits, site prep, utility connections, and construction. The $250K–$350K quotes you may see online typically exclude several of these line items.
These ranges reflect Seattle and King County construction costs in 2026. Your actual cost depends on site-specific factors like slope, utilities, and finish level. For a number tailored to your property, request a free feasibility study and we'll run the analysis.
ADU Cost Per Square Foot in Seattle
In Seattle and King County, total ADU project costs start at $400 per square foot for basic conversions and reach $650+ per square foot for new DADU construction with premium finishes. This includes design, permitting, construction, and utility connections. Seattle's higher labor costs and permit fees mean these numbers sit above the statewide average.
| ADU Size | Basic Finish | Mid-Range | High-End |
|---|---|---|---|
| 400 sq ft | $180,000 – $230,000 | $230,000 – $280,000 | $280,000 – $330,000 |
| 600 sq ft | $270,000 – $340,000 | $340,000 – $410,000 | $410,000 – $490,000 |
| 800 sq ft | $360,000 – $450,000 | $450,000 – $540,000 | $540,000 – $640,000 |
| 1,000 sq ft | $450,000 – $560,000 | $560,000 – $650,000 | $650,000 – $750,000+ |
| 1,200 sq ft | $540,000 – $660,000 | $660,000 – $780,000 | $780,000 – $900,000+ |
Based on 2026 Seattle/King County total project costs at $400–$650/sq ft. Includes design, permitting, construction, and utility connections. Site work and landscaping may be additional.
Not sure which size makes sense for your lot? Get a free cost estimate for your specific property — we'll analyze your lot constraints and recommend the right size.
Permit & Soft Costs: What You'll Pay Before Construction
Beyond construction, ADU permit costs and pre-construction soft costs typically add $20,000 to $50,000 for Seattle city properties. King County DPER permits (unincorporated areas) run $10,000–$25,000 in permit fees alone, with higher total soft costs for rural lots. Here's the full breakdown:
Hard Costs (Construction)
- Foundation & site prep
- Framing & structural
- Roofing & exterior siding
- Plumbing (rough & finish)
- Electrical (rough & finish)
- HVAC / mini-split system
- Insulation & drywall
- Interior finishes (flooring, cabinets, counters)
- Windows & doors
- Bathroom fixtures
- Kitchen appliances
- Painting & trim
Soft Costs (Pre-Construction)
- Architectural design: $8,000 – $20,000
- Site survey: $1,500 – $4,000
- Structural engineering: $3,000 – $7,000
- Seattle DCI permit fees: $8,000 – $20,000
- King County DPER fees: $10,000 – $25,000
- Utility connections (SPU/SCL): $10,000 – $25,000
- Geotechnical report: $3,000 – $7,000 (if needed)
- SMA review: $5,000 – $15,000 (waterfront properties)
What Would an ADU Cost on Your Lot?
Every property is different. Get a free feasibility report with a cost estimate based on your lot size, zoning, and site conditions.
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(206) 222-2898Cost Difference: Seattle vs Unincorporated King County
Seattle (SPU Sewer)
Unincorporated King County (DPER)
Rural King County properties on septic add significant cost. Unincorporated areas use DPER for permitting, not Seattle DCI.
Factors That Affect ADU Cost
ADU Type
High impactDADUs are the most expensive due to full new construction. Garage and basement conversions have lower totals but similar cost-per-square-foot once all systems are included.
Size
High impactCost scales roughly per square foot. A 600 sq ft ADU costs ~60% of a 1,000 sq ft unit. Seattle labor costs make every square foot count.
Site Conditions
High impactSlope, soil quality, tree removal, access constraints — Seattle's hilly topography means difficult sites add $15K–$60K+.
Seattle Labor Costs
High impactSeattle wages are among the highest in Washington. Skilled trade labor runs 20–30% above statewide averages, which flows directly into construction costs.
Finish Level
Medium impactBuilder-grade vs high-end finishes (quartz counters, hardwood floors, custom cabinets) can add $50K–$100K+ to interior costs.
Utility Infrastructure
High impactCity sewer and SPU water connections required in Seattle. Electrical service capacity upgrades may be needed. SCL connection fees apply.
SMA Zone
Medium impactProperties within 200 ft of Ordinary High Water Mark (Shoreline Management Act) add $5K–$15K in review costs, plus potential design constraints.
Parking Requirements
Low–Med impactWithin ½ mile of Link Light Rail or RapidRide: no parking required. Outside transit zone: one space may be required (~$8K–$15K).
Ready to get a real number?
We'll analyze your specific lot and give you a cost range — free, no strings attached.
Ways to Reduce ADU Costs
Convert an existing structure (garage or basement) — lower total cost due to smaller buildable size
Build within ½ mile of Link Light Rail or RapidRide to eliminate parking requirement — saves $8K–$15K
Choose a smaller footprint (600–800 sq ft) — adequate for most Seattle rental markets at $2,000–$3,500/mo
Claim the 30% federal energy efficiency tax credit through sustainable construction (Inflation Reduction Act)
Finance with a HELOC or ADU-specific construction loan at favorable rates
Phase construction: shell first, finish interior in a second phase
Use Seattle DCI's pre-application conference to identify issues before full permit submission
Bundle permit + build with one team (like ours) to avoid duplicated costs — see our 3-step process
ADU Return on Investment in Seattle
A typical Seattle ADU investment generates meaningful returns through rental income and property value appreciation:
$2,000–$3,500
Monthly ADU rental income
$24,000–$42,000
Annual gross income
15–25%
Typical property value increase
Example: A DADU costing $600,000 renting at $2,800/month generates $33,600/year gross — a 5.6% gross yield before expenses. When combined with Seattle's strong appreciation and Washington's zero state income tax, the long-term math is compelling.
Many Seattle ADU owners report 12–18 year payback periods on rental income alone, before accounting for property appreciation.
Want to see the ROI math for your specific property? Our free feasibility study includes a rental income projection based on your neighborhood and ADU size. For full pricing, see our pricing breakdown.
Continue Reading
How to Finance Your ADU
HELOCs, construction loans, and ADU-specific financing options for Washington homeowners.
ADU Construction Timeline
Month-by-month breakdown from design to move-in for Seattle ADU projects.
Is an ADU Worth It?
ROI analysis, rental income projections, and property value impact for Seattle homeowners.
How to Hire an ADU Contractor
What to look for, questions to ask, and red flags when choosing your ADU builder.
ADU Cost Per Square Foot Guide
Detailed per-square-foot analysis by ADU type, finish level, and Washington region.
Prefab vs Site-Built ADU
Full cost comparison, pros and cons, and which option works best for your lot.
Ready to Get a Real Number for Your Property?
These are general cost ranges. Your ADU project will depend on your specific lot, zoning, utilities, and design preferences. The fastest way to get a realistic budget is to start with our free feasibility study — or contact us directly.
Related Guides
Last updated: June 2026. Costs are estimates based on Seattle and King County market conditions. Actual costs vary by project scope and site conditions.
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