Seattle ADU Costs

Your 2025 Guide to ADU & DADU Costs in Seattle

If you’re considering adding an Accessory Dwelling Unit (ADU) or a Detached Accessory Dwelling Unit (DADU) to your Seattle property, your first question is likely: “How much will it actually cost?” In a complex construction market like King County, getting a straight answer can be challenging.

As experienced builders, we believe in transparency. The realistic, “all-in” cost for a quality, turnkey DADU project in Seattle for 2025 is between $400,000 and $600,000. On the Eastside, in areas like Bellevue and Redmond, that range is typically higher, from $450,000 to $650,000.   

This guide breaks down exactly where that money goes, why online estimates can be misleading, and what key factors will shape your final budget.

Why Online Cost Estimates Can Be Deceiving

A quick search might show DADU cost estimates as low as $125,000 to $350,000. These numbers are often misleading because they typically only cover “hard costs” (basic construction) and leave out major expenses. A realistic budget must account for the complete project, which includes:   

• Soft Costs: Architecture, engineering, and land surveys.

• Permitting & Fees: City permits, utility connection fees, and sales tax.

• Site Preparation: Excavation, grading, and foundation work.

• Project Management: The cost of a general contractor to manage the entire process.

Be wary of any builder who claims they can complete your project for under six figures; it’s simply not feasible for a legal, quality DADU in today’s market.   

A Realistic DADU Budget Breakdown: Where Your Investment Goes

Building a DADU is a multi-phase process. Here’s a forensic look at how a typical budget of around $450,000 for an 800-square-foot DADU in Seattle is allocated.  

Phase 1: Pre-Construction & Design (Soft Costs: $14,000 – $31,000+) This is the essential planning phase that sets your project up for success.

• Architectural Design & Plans: $5,000 – $15,000.   

   

• Structural Engineering: $2,000 – $4,000.   

   

• Land / Site Survey: $2,000 – $3,500.   

   

• Specialized Reports: Depending on your property, you may need reports from a geotechnical engineer (for soil stability) or an arborist (for trees), which can add several thousand dollars each.   

   

Phase 2: Permits & Regulatory Fees ($43,000 – $81,000+) Navigating city and county requirements is a significant cost center.

• City of Seattle Permit Fees: Can range from $20,000 to $40,000 depending on the project’s scope. Note that for 2025, Seattle’s construction permit fees increased by an average of 12%, with some larger projects seeing a 19% hike.   

   

• Utility Connection Fees: Hooking up to water, sewer, and electrical mains can cost between $10,000 and $30,000.   

   

• King County Sewer Capacity Charge: A fee billed over 15 years for connecting to the county’s wastewater system.   

   

• Sales Tax: Applied to both materials and labor, this can easily add $23,000 or more to the budget.   

   

Phase 3: Construction (Hard Costs: $205,000 – $322,000+) This is the largest part of your budget, covering all physical construction.

• Site Prep, Excavation & Foundation: A new foundation alone can cost $30,000 to $50,000, with costs increasing for sloped or challenging lots.   

   

• Structure & Exterior: Framing, lumber, sheathing, roofing, siding, windows, and doors. The core structure can cost over $50,000.   

   

• Utilities Rough-in (Plumbing, Electrical, HVAC): Installing the essential systems inside the walls is a major expense, often totaling $30,000 to $50,000.   

   

• Interior Finishes: This includes everything from insulation and drywall to flooring, paint, trim, cabinetry, and countertops. This is where your personal style choices have the biggest budget impact.   

   

• Kitchen & Bath: These are the most expensive rooms per square foot, with costs for fixtures, tile, and vanities adding up quickly.   

   

Phase 4: Project Management & Final Touches ($48,000 – $87,000+)

• General Contractor Fee: Typically 10-20% of the total project cost, this covers the management, scheduling, and supervision of the entire build.   

   

• Landscaping & Exterior Work: Walkways, patios, and planting to integrate the DADU with your property can cost $5,000 to $12,000 or more.   

   

• Contingency Fund: It is essential to set aside at least 10-15% of your total budget for unforeseen issues that inevitably arise during construction.   

   

The 3 Key Factors That Drive Your Final DADU Cost

1. Your Property (Site Conditions): This is the biggest variable. A flat, clear lot with easy access is ideal. Costs escalate quickly with challenges like steep slopes (requiring complex foundations), poor soil quality, large trees, or long distances to utility connections.   

    

2. Your Plans (Size & Layout): While a larger DADU costs more overall, it typically has a lower cost-per-square-foot. This is because every DADU, regardless of size, has expensive fixed costs like a kitchen, a bathroom, and design fees. Spreading these fixed costs over more square footage makes the project more efficient. A simple, rectangular footprint is also more cost-effective than a complex design with many corners and angles.   

    

3. Your Style (Finishes & Features): The quality of materials you choose creates a huge cost variance. Standard, builder-grade finishes will keep you at the lower end of the budget range. Opting for luxury items like custom cabinetry, quartz countertops, hardwood floors, and high-end appliances will significantly increase the final price.   

    

A Warning About Cost-Per-Square-Foot

Many people try to budget using a simple cost-per-square-foot (CPSF) metric, but for DADUs, this is unreliable. Because of the high fixed costs mentioned above, a 400 sq. ft. studio will have a much higher CPSF than a 1,000 sq. ft. unit. For a custom, site-built DADU in Seattle, a realistic CPSF is between $500 and $700, with some recent projects reporting even higher figures.   

New 2025 Regulations: A Game-Changer for Homeowners

The good news for 2025 is the full implementation of Washington State House Bill 1337, which makes building a DADU more attractive than ever. Key benefits include:  

• No Owner-Occupancy Required: You can now rent out both your main house and the DADU, turning your property into a powerful investment vehicle.

• Two ADUs Allowed: You can now have up to two ADUs on a single-family lot (e.g., one attached and one detached).

• Reduced Parking Requirements: If your property is near a major transit stop, you are no longer required to provide off-street parking for the DADU, saving you thousands in construction costs.

Partner with an Expert to Navigate the Process

Building a DADU is one of the most significant investments you can make in your property. The process is complex, and the financial stakes are high. Working with an experienced, professional builder is the best way to ensure your project is completed on time, on budget, and to the highest standards of quality.

Ready to explore the potential of your property? Contact Seattle ADU Builders today for a comprehensive consultation and a detailed estimate tailored to your vision.