One of the first questions every homeowner asks when considering an accessory dwelling unit is: how much is this going to cost? The answer depends on the type of ADU you're building, your location in Washington state, the size and finishes you choose, and your site conditions. But we can give you solid ballpark numbers based on real project data from builders across the Puget Sound region.
Average ADU Costs by Type
Washington homeowners generally choose between three types of ADUs. Here's what each typically costs in 2026:
| ADU Type | Typical Cost Range | Cost per Sq Ft | Timeline |
|---|---|---|---|
| Attached ADU (AADU) | $150,000 - $250,000 | $250 - $400 | 4 - 8 months |
| Detached ADU (DADU) | $200,000 - $400,000 | $300 - $450 | 6 - 14 months |
| Garage Conversion | $80,000 - $150,000 | $200 - $350 | 3 - 6 months |
Attached ADUs (basement conversions, bump-outs, above-garage units) are the most affordable option because they leverage your home's existing structure, foundation, and utility connections. A basement conversion on the lower end might come in around $150,000, while a full addition with a new foundation can push past $250,000.
Detached ADUs (DADUs) are standalone backyard cottages that require their own foundation, walls, roof, and utility connections. The wide cost range reflects differences in size (400 to 1,000+ square feet), finish level, and site complexity. Most DADUs in the Seattle area land between $250,000 and $350,000 for a 600-800 square foot unit with mid-range finishes.
Garage conversions offer the lowest entry point. Since the shell already exists, you're primarily paying for insulation, interior framing, plumbing, electrical, HVAC, and finishes. A basic single-car garage conversion can come in under $100,000, though adding a bathroom and kitchen will push costs higher.
Where the Money Goes: Cost Breakdown
Understanding where your budget goes helps you make smarter decisions about where to invest and where to economize. Here's a typical cost breakdown for a mid-range DADU project:
- Construction (hard costs): $250 - $450 per square foot. This is the biggest line item and includes foundation, framing, roofing, siding, insulation, drywall, flooring, cabinetry, fixtures, and finishes. Material choices drive much of the variance here.
- Permits and fees: $5,000 - $15,000. Seattle permit fees for a DADU typically run $8,000 to $12,000, including plan review, building permits, and inspection fees. Tacoma and smaller cities tend to be on the lower end. Impact fees, where applicable, can add more.
- Design and architecture: $10,000 - $30,000. A custom design from an architect costs more, while pre-designed plans or a design-build firm's standard layouts will be cheaper. Many builders include design in their project price.
- Utility connections: $10,000 - $30,000. Running sewer, water, and electrical service to a detached unit is a significant cost. Distance from the main house, soil conditions, and whether you need to trench through hardscape all affect this number.
- Site work and landscaping: $5,000 - $15,000. Grading, drainage, access paths, fencing, and restoring your yard after construction. Sloped lots or sites with limited equipment access will cost more.
Factors That Affect Your Total Cost
Size and Layout
Bigger costs more, obviously, but it's not linear. A 400 square foot ADU doesn't cost half as much as an 800 square foot one, because the expensive components (kitchen, bathroom, utility hookups, permits) are roughly the same regardless of size. Per-square-foot costs actually decrease as the unit gets larger.
Site Conditions
A flat, accessible lot with good soil and a short run to utilities is the cheapest to build on. Sloped lots may require retaining walls or engineered foundations. Lots with limited access (no alley, narrow side yards) increase labor costs because equipment and materials are harder to deliver. Mature trees in the build zone may require an arborist assessment and root protection plan.
Finish Level
The gap between "builder grade" and "custom" finishes is enormous. Standard laminate countertops, LVP flooring, and basic fixtures might add $30 per square foot to your finish costs. Quartz countertops, hardwood floors, custom cabinetry, and premium appliances can push that to $80+ per square foot.
Permitting Jurisdiction
Permit costs and timelines vary by city. Seattle's SDCI (Seattle Department of Construction and Inspections) has streamlined its ADU permitting in recent years, but reviews still take 4 to 8 weeks. Tacoma, Bellevue, and other cities each have their own fee structures and review timelines.
ADU Cost Comparison by City
| City | Avg. DADU Cost (600 sq ft) | Permit Fees | Permit Timeline |
|---|---|---|---|
| Seattle | $280,000 - $380,000 | $8,000 - $12,000 | 4 - 8 weeks |
| Tacoma | $220,000 - $320,000 | $5,000 - $9,000 | 3 - 6 weeks |
| Bellevue | $300,000 - $420,000 | $10,000 - $15,000 | 6 - 10 weeks |
| Olympia | $200,000 - $300,000 | $4,000 - $8,000 | 3 - 5 weeks |
| Everett | $220,000 - $330,000 | $5,000 - $10,000 | 4 - 7 weeks |
Bellevue tends to be the most expensive due to higher land values, stricter design requirements, and a more competitive contractor market. Tacoma offers the best value for many homeowners, with lower construction costs and a growing pool of experienced DADU builders.
Financing Your ADU
Most homeowners don't pay for an ADU entirely out of pocket. Here are the most common financing options:
- Home Equity Line of Credit (HELOC): The most popular option. You borrow against your existing home equity at variable rates, typically 7-9% in 2026. Best for homeowners with significant equity and good credit.
- Construction loan: A short-term loan that converts to a mortgage once the ADU is complete. Rates are slightly higher than traditional mortgages, but you only pay interest on funds as they're drawn. Good for larger projects where you need to finance the full build.
- Cash-out refinance: Refinance your primary mortgage for a higher amount and use the difference to fund the ADU. This makes sense if your current mortgage rate is close to today's rates and you have substantial equity.
- ADU-specific loan programs: Some local credit unions and community development financial institutions (CDFIs) in Washington offer loan products specifically designed for ADU construction. These sometimes come with lower down payment requirements or favorable terms for owner-occupants.
The ROI Equation: Is an ADU Worth It?
Thanks to HB 1337, the investment case for ADUs in Washington has never been stronger. With owner-occupancy requirements eliminated, you can rent both the main house and the ADU, or sell the property to an investor who will. A well-built DADU in Seattle can rent for $1,800 to $2,800 per month depending on size, location, and finishes. That translates to $21,600 to $33,600 per year in gross rental income.
Most appraisers and real estate agents estimate that a quality ADU adds 20-30% of its construction cost to the property's market value. A $300,000 DADU might add $60,000 to $90,000 in appraised value immediately, with the rental income stream providing ongoing returns. In a high-demand market like Seattle, the payback period for a well-executed ADU is typically 8 to 15 years.
Ready to get accurate pricing for your specific project? Request free quotes from multiple vetted ADU builders in your area and compare real numbers side by side.