How Much Does It Cost to Build a DADU in King County? (2026 Guide)

What Does a DADU Actually Cost in King County?

Building a detached accessory dwelling unit in King County is a significant investment — but one that typically pays for itself through rental income and property value increases. In 2026, the all-in cost for a typical 800 square foot DADU ranges from $400,000 to $650,000, depending on location, site conditions, design complexity, and finishes.

On a per-square-foot basis, expect to pay $500–$700 per square foot for a custom, site-built DADU including all soft costs, permits, construction, and management fees. Eastside locations like Bellevue and Redmond tend to run 10–15% higher than Seattle proper.

Complete Cost Breakdown

Understanding where your money goes helps you make informed decisions about where to invest and where to save. Here’s how a typical 800 square foot DADU budget breaks down:

Design and Engineering: $14,000–$31,000

Architectural design: $8,000–$20,000. Custom designs cost more than adapting a stock plan, but result in a better fit for your lot and needs.
Structural engineering: $3,000–$6,000. Required for permit approval.
Site survey: $1,500–$3,000. Establishes property boundaries, elevation, and existing conditions.
Geotechnical report: $1,500–$3,000. Required in many King County areas to assess soil conditions for foundation design.

Permits and Fees: $43,000–$81,000

Building permit fees: $15,000–$35,000. King County permit fees increased 12–19% in 2025.
Impact fees: $15,000–$30,000. School, transportation, and park impact fees vary by jurisdiction.
Utility connection fees: $8,000–$16,000. Sewer connection, water meter, and electrical service.
Plan review: $5,000–$10,000. Included in some jurisdictions, separate in others.

Construction: $205,000–$322,000

Site preparation: $10,000–$25,000. Clearing, grading, and excavation.
Foundation: $20,000–$40,000. Slab-on-grade is cheapest; crawl space and basement add cost.
Framing and structure: $35,000–$55,000.
Roofing: $10,000–$18,000.
Plumbing: $15,000–$25,000.
Electrical: $12,000–$22,000.
HVAC: $10,000–$20,000. Mini-split heat pumps are the most common choice.
Insulation and drywall: $15,000–$25,000.
Interior finishes: $30,000–$50,000. Flooring, cabinetry, countertops, fixtures.
Exterior finishes: $20,000–$35,000. Siding, windows, doors, paint.
Utilities to structure: $10,000–$30,000. Running sewer, water, and electrical from the main house or street.

Project Management and Extras: $48,000–$87,000

General contractor overhead and profit: $30,000–$55,000. Typically 15–20% of construction costs.
Landscaping restoration: $5,000–$15,000. Repairing your yard after construction.
Contingency: $10,000–$15,000. Always budget 5–10% for unexpected issues.
Temporary utilities and dumpsters: $3,000–$5,000.

Factors That Increase DADU Costs

Several factors can push your project toward the higher end of the range — or beyond it:

Sloped lots: Hillside construction requires engineered foundations and retaining walls, adding $30,000–$50,000.
Poor soil conditions: Soft or wet soils may require pilings or over-excavation.
Tree removal: King County has strict tree protection regulations. Removal permits and replacement requirements add cost and time.
Long utility runs: If your DADU site is far from existing utility connections, trenching and piping costs increase significantly.
Septic systems: Properties on septic may need system evaluation and potentially upgrades, adding $10,000–$30,000+.
Premium finishes: High-end kitchens, bathrooms, and materials can add 20–50% to interior costs.
Eastside premium: Construction costs in Bellevue, Redmond, and Kirkland typically run 10–15% higher than Seattle.

Ways to Reduce DADU Costs

Smart planning can bring costs down without sacrificing quality:

Simple footprint: Rectangular designs are significantly cheaper to build than L-shapes or complex layouts.
Standard sizes: Designing to standard lumber dimensions reduces waste and labor.
Slab foundation: Where soil conditions allow, slab-on-grade is the most affordable foundation option.
Moderate finishes: Mid-range cabinets, countertops, and fixtures look great and cost 30–40% less than premium options.
Efficient layout: Minimizing plumbing walls by clustering the kitchen and bathroom saves on piping and labor.
Stock plans: Adapting a proven DADU floor plan costs less than a fully custom design.

DADU Cost vs. Return on Investment

Despite the significant upfront cost, DADUs in King County typically deliver strong returns:

Rental income: A well-located DADU can generate $1,500–$3,000+ per month, potentially $18,000–$36,000+ annually.
Property value increase: DADUs typically add 20–30% to property value, often exceeding the construction cost.
Break-even timeline: Most King County DADU owners recoup their investment within 10–15 years through rental income alone — faster when factoring in property appreciation.

For a detailed investment analysis, check our DADU ROI guide.

Get an Accurate Estimate for Your Property

Every lot is different, and the cost range above reflects that variety. The best way to get an accurate number is a site-specific feasibility assessment that evaluates your lot conditions, utility access, zoning requirements, and design goals.

APEX DADU provides free initial consultations for King County homeowners considering a DADU project. We’ll walk your property, review the zoning, and give you a realistic cost range before you commit to anything.

Request your free DADU cost estimate