DADU Construction Cost Breakdown: Where Your Money Goes

Understanding Where Your DADU Budget Goes

When you’re quoted $400,000–$650,000 for a DADU in King County, that number can feel abstract. Breaking it down into specific line items helps you understand what you’re paying for, where you have flexibility, and where costs are fixed. This guide walks through every major cost category so you can plan with confidence.

We’ll use a typical 800-square-foot, single-story DADU as our reference — the most common configuration built in King County. Costs scale up or down based on size, complexity, and finishes.

Soft Costs: Before Construction Starts

Before a single shovel hits the ground, you’ll spend $57,000–$112,000 on planning, design, and permitting.

Design and Engineering ($14,000–$31,000)

• Architectural design: $8,000–$20,000. Custom designs cost more but fit your lot perfectly. Stock plan adaptations save money.
• Structural engineering: $3,000–$6,000. Required for every DADU permit in King County.
• Site survey: $1,500–$3,000. Establishes property boundaries and elevations. Non-negotiable.
• Geotechnical report: $1,500–$3,000. Required when soil conditions or slopes warrant investigation.

Permits and Fees ($43,000–$81,000)

• Building permit: $15,000–$35,000. King County fees are based on project valuation.
• Impact fees: $15,000–$30,000. School, transportation, and park impact fees vary by jurisdiction.
• Utility connections: $8,000–$16,000. Sewer connection, water meter, and electrical service fees. See our sewer vs. septic guide for details.
• Plan review: $5,000–$10,000. Some jurisdictions bundle this with the building permit; others charge separately.

Site Work: Preparing the Ground

Site preparation costs vary dramatically based on your lot’s conditions. Budget $10,000–$25,000 for a typical project.

• Clearing and grading: $3,000–$10,000. Removing vegetation, leveling the build area, and establishing drainage patterns.
• Excavation: $5,000–$12,000. Digging for the foundation, utility trenches, and drainage systems.
• Tree protection: $1,000–$3,000. King County requires protection fencing around significant trees near the construction zone.
• Erosion control: $1,000–$3,000. Silt fencing, straw wattles, and drainage management during construction.

Site Cost Multipliers

• Slopes: Add $20,000–$50,000 for retaining walls and engineered foundations on sloped lots.
• Poor access: If heavy equipment can’t reach the build site, manual labor increases costs 15–25%.
• Rock removal: Hitting bedrock during excavation adds $5,000–$15,000 for removal.

Foundation: $20,000–$40,000

Your foundation choice significantly impacts both cost and long-term performance:

• Slab-on-grade: $20,000–$28,000. The most affordable option. Works well on flat lots with stable soil.
• Crawl space: $25,000–$35,000. Provides access to plumbing and mechanical systems. Better for sloped sites.
• Basement/daylight basement: $35,000–$55,000. Adds usable (unheated) space below the DADU. Best on sloped lots where the grade naturally exposes one wall.

Learn more about foundation options in our foundation guide.

Framing and Structure: $35,000–$55,000

• Floor framing: $5,000–$10,000 (not needed for slab-on-grade)
• Wall framing: $12,000–$20,000. Includes exterior and interior walls, headers, and blocking.
• Roof framing: $10,000–$18,000. Simple gable roofs cost less than hip roofs or complex designs.
• Sheathing: $5,000–$8,000. Plywood or OSB applied to walls and roof.
• Windows and exterior doors: $8,000–$15,000. Quality windows are essential for Pacific Northwest weather and energy code compliance.

Mechanical Systems: $37,000–$67,000

Plumbing, electrical, and HVAC are where small DADUs face disproportionate costs — a 400-square-foot unit needs the same basic systems as an 800-square-foot unit.

Plumbing ($15,000–$25,000)

• Water supply lines from main house or street
• Drain, waste, and vent piping
• Sewer connection or septic tie-in
• Water heater (tankless units are popular in DADUs)
• Kitchen and bathroom rough-in

Electrical ($12,000–$22,000)

• New subpanel or separate service panel
• Branch circuits throughout the DADU
• Lighting rough-in
• Smoke and CO detector wiring
• Exterior lighting and outlet circuits

HVAC ($10,000–$20,000)

• Mini-split heat pump: $8,000–$15,000. The standard choice for King County DADUs — efficient, quiet, and provides both heating and cooling.
• Ducted system: $12,000–$20,000. More expensive but provides more even air distribution.
• Bathroom exhaust fans: $500–$1,000
• Range hood: $300–$800

Exterior Finishes: $20,000–$35,000

• Siding: $8,000–$18,000. Fiber cement (HardiePlank) is the most common choice in King County. Cedar and wood-look options cost more.
• Roofing: $6,000–$12,000. Architectural shingles are standard. Metal roofing adds $3,000–$5,000.
• Exterior paint or stain: $2,000–$4,000
• Gutters and downspouts: $1,500–$3,000. Essential in the Pacific Northwest.
• Exterior trim and details: $2,000–$4,000

Interior Finishes: $30,000–$50,000

This is where you have the most control over costs. The range between builder-grade and premium finishes is significant.

• Insulation and drywall: $8,000–$14,000
• Interior paint: $2,000–$4,000
• Flooring: $4,000–$10,000. LVP (luxury vinyl plank) is the most popular choice — durable, waterproof, and cost-effective.
• Kitchen cabinetry and countertops: $6,000–$12,000. Custom cabinets and quartz countertops push this higher.
• Bathroom tile and fixtures: $4,000–$8,000
• Interior doors and trim: $2,000–$4,000
• Appliances: $3,000–$6,000. Compact and standard-size options for DADU kitchens.
• Lighting fixtures: $1,500–$3,000

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

• General contractor overhead and profit: $30,000–$55,000. Typically 15–20% of construction costs. This covers project management, scheduling, quality control, insurance, and warranty.
• Landscaping restoration: $5,000–$15,000. Your yard will need repair after construction. Plan for new sod, planting, and path restoration.
• Contingency: $10,000–$15,000. Budget 5–10% for the unexpected — hidden conditions, material price changes, or design adjustments.
• Temporary utilities and waste: $3,000–$5,000. Construction power, portable toilets, and dumpster rental.

Where to Save Without Sacrificing Quality

• Foundation: Choose slab-on-grade when soil conditions allow — saves $10,000–$20,000 vs. crawl space.
• Roof design: Simple gable roofs save $3,000–$5,000 vs. complex designs.
• Finishes: Mid-range cabinets, LVP flooring, and standard-grade fixtures deliver 80% of the look at 50% of the cost.
• Layout efficiency: Back-to-back plumbing (kitchen and bathroom sharing a wall) saves $2,000–$4,000 in piping.

For the full cost picture and ROI analysis, see our comprehensive DADU cost guide.

APEX DADU Provides Transparent Pricing

We believe you should know exactly where every dollar goes. APEX DADU provides detailed, line-item cost breakdowns for every project — no hidden fees, no surprise change orders. You’ll know your total investment before construction begins.

Request a detailed DADU cost estimate from APEX DADU