How Much Does an ADU Cost to Build in Seattle in 2026?

# How Much Does an ADU Cost to Build in Seattle in 2026?

Building an accessory dwelling unit in Seattle or King County is one of the best investments a homeowner can make right now, but the price tag varies widely depending on what you build and where you build it. A detached ADU (DADU) in Seattle typically costs between $200,000 and $400,000 in 2026. An attached ADU runs $100,000 to $250,000. Those ranges are broad because the variables are real: lot conditions, unit size, foundation type, finishes, and whether you build to standard code or to a higher-performance standard like Passive House.

This guide breaks down exactly where that money goes so you can plan with actual numbers, not guesses.

Aaron Hundtofte, founder of Love Construction in SeaTac, has built ADUs and DADUs across King County for years. "The most common mistake homeowners make is budgeting based on a per-square-foot number they found online without understanding what drives the real cost on their specific lot," he says. "Site access, utility connections, and foundation work can swing the total by $50,000 or more."

Detached ADU (DADU) Costs in Seattle: $200,000 to $400,000

A detached ADU is a standalone structure, separate from your primary home. It needs its own foundation, its own roof, its own utility connections. That independence is what makes DADUs more expensive than attached units, but it also makes them more versatile and more valuable as rental properties.

Here is where the money goes on a typical 600- to 800-square-foot DADU in the Seattle metro:

| Cost Category | Typical Range | Notes | |---|---|---| | Design and engineering | $15,000 - $30,000 | Architectural plans, structural engineering, energy modeling | | Permitting | $8,000 - $20,000 | Seattle DCI fees, impact fees (varies by jurisdiction) | | Site preparation | $10,000 - $40,000 | Excavation, grading, tree removal, access path | | Foundation | $15,000 - $35,000 | Slab-on-grade is cheapest; crawlspace or full basement adds cost | | Framing and structure | $30,000 - $60,000 | Wood frame standard; advanced framing reduces thermal bridging | | Mechanical (HVAC, plumbing, electrical) | $25,000 - $50,000 | Heat pump systems, plumbing runs, electrical panel | | Insulation and air sealing | $8,000 - $20,000 | Standard code vs Passive House envelope | | Interior finishes | $20,000 - $50,000 | Cabinets, countertops, flooring, fixtures | | Exterior finishes | $15,000 - $30,000 | Siding, roofing, windows, doors | | Utility connections | $10,000 - $25,000 | Sewer, water, electrical service from street | | Landscaping and site restoration | $5,000 - $15,000 | Restoring the yard after construction |

Total range: $161,000 - $375,000, with most projects in King County landing between $220,000 and $320,000 for a well-finished unit.

Attached ADU Costs in Seattle: $100,000 to $250,000

An attached ADU shares a wall, foundation, or roofline with the primary house. Common configurations include basement conversions, garage conversions, and above-garage additions. Because you are building within or onto an existing structure, several major cost categories shrink or disappear.

Basement ADU conversions typically run $100,000 to $180,000 in Seattle. Garage conversions fall in a similar range. Above-garage additions cost more because they require structural upgrades to the existing garage, usually $150,000 to $250,000.

The savings over a detached unit come from shared foundation, shared roofline (in some configurations), shorter utility runs, and less site work. The tradeoff is less privacy, shared walls, and potentially lower rental income compared to a fully independent DADU.

What Drives ADU Cost Up (or Down)

Lot Access

If a concrete truck or excavator cannot reach your backyard without removing a fence, a tree, or navigating a narrow side yard, site access costs rise quickly. Crane lifts for modular components or tight-access excavation can add $10,000 to $30,000. Flat lots with a wide side yard or alley access are the least expensive to build on.

Foundation Type

A slab-on-grade foundation is the most affordable option, typically $15,000 to $20,000. A crawlspace adds $5,000 to $10,000. A full basement foundation can add $30,000 to $50,000 but also gives you additional usable square footage below grade.

Unit Size

Seattle allows ADUs up to 1,000 square feet in most residential zones. Cost per square foot decreases as unit size increases because fixed costs (permitting, design, utility connections) are spread over more area. A 400-square-foot studio might cost $250 per square foot while an 800-square-foot two-bedroom might cost $300 per square foot total, but the per-foot number is lower.

Finish Level

Builder-grade finishes (laminate counters, vinyl plank flooring, basic fixtures) keep costs at the lower end. Mid-range finishes (quartz counters, hardwood floors, semi-custom cabinets) add $15,000 to $30,000. High-end custom finishes can push well beyond $400,000 total.

Passive House Certification

Building your ADU to Passive House standards adds 10% to 15% to construction cost. On a $250,000 DADU, that is $25,000 to $37,500 more. The premium pays for triple-pane windows, continuous exterior insulation, an airtight building envelope verified by blower door testing, and a heat recovery ventilation (HRV) system.

The return: heating and cooling costs drop by up to 90%. A standard ADU might cost $100 to $150 per month to heat in a Seattle winter. A Passive House ADU costs $10 to $20. Over 20 years, the energy savings alone cover the upfront premium, and the comfort difference is immediate. No drafts, consistent temperature in every room, and continuously filtered fresh air.

Love Construction is one of the few Passivehaus-certified builders in the Seattle area. Every ADU we build can be designed to meet the standard.

Permitting Costs and Timeline in Seattle and King County

Seattle (City of Seattle DCI)

Seattle's Department of Construction and Inspections handles ADU permits. As of 2026:

• • Pre-application conference: $200 to $400 (optional but recommended)
• • Building permit fees: $5,000 to $12,000 depending on project value
• • Plan review: 4 to 8 weeks for standard review; 2 to 4 weeks for expedited review (additional fee)
• • Impact fees: Seattle currently waives impact fees for ADUs in most single-family zones
• • Total permit timeline: 6 to 12 weeks from application to permit issuance

Unincorporated King County and Other Jurisdictions

SeaTac, Renton, Burien, Tukwila, Kent, and other King County cities each have their own permitting processes. Fees and timelines vary, but most jurisdictions have streamlined ADU permitting in response to Washington State's 2023 ADU legislation (HB 1337), which required cities to allow at least two ADUs per residential lot.

Budget $8,000 to $20,000 total for permitting costs across any King County jurisdiction, including plan review fees, building permit fees, and utility connection fees.

Construction Timeline

Once permits are in hand, construction typically takes:

• • Attached ADU (basement or garage conversion): 3 to 5 months
• • Detached ADU (new construction): 5 to 8 months
• • Passive House ADU: Add 2 to 4 weeks for air sealing verification and HRV commissioning

Total project timeline from first design meeting to move-in: 9 to 14 months for most detached ADUs in Seattle.

Washington State ADU Law Changes (HB 1337)

Washington's 2023 ADU legislation changed the math for homeowners across King County. Key provisions that took effect in 2024 and 2025:

• • Cities with populations over 25,000 must allow at least two ADUs per residential lot (one attached, one detached, or two detached)
• • Owner-occupancy requirements are eliminated
• • Off-street parking requirements are reduced or eliminated near transit
• • Impact fees for ADUs are capped
• • Design standards cannot be more restrictive than those applied to the primary dwelling

These changes mean more lots qualify for ADU construction than ever before, and the regulatory burden is lower than it was even two years ago. If you were told your lot could not support an ADU before 2024, it is worth revisiting the question.

How to Budget for Your ADU Project

A practical budgeting approach for a Seattle-area ADU:

1. Get a site assessment first. Lot conditions, access, and utility locations determine 30% of your final cost. A $500 site visit saves you from budgeting based on assumptions. 2. Design to your budget, not the other way around. If your budget is $250,000, design a unit that fits within that number. Do not design a $350,000 unit and then try to value-engineer it down. 3. Hold a 10% contingency. Underground surprises, material price changes, and scope adjustments happen. A $25,000 contingency on a $250,000 project is reasonable. 4. Finance strategically. Home equity lines of credit (HELOCs), construction loans, and cash-out refinances are the most common ADU financing methods. Some homeowners use rental income projections to qualify for the loan.

Love Construction provides free consultations and detailed project estimates for ADU construction anywhere in King County. Call (206) 339-2718 or request a quote online.

Frequently Asked Questions

How much does a detached ADU cost in Seattle in 2026? Most detached ADUs (DADUs) in Seattle cost between $200,000 and $400,000 depending on size, site conditions, foundation type, and finish level. The average for a well-finished 600- to 800-square-foot unit is $220,000 to $320,000.

How much does an attached ADU cost in Seattle? Attached ADUs, including basement conversions and garage conversions, typically cost $100,000 to $250,000. Basement conversions are at the lower end; above-garage additions are at the higher end.

How long does it take to build an ADU in Seattle? Total project timeline from initial design through move-in is 9 to 14 months for most detached ADUs. Construction alone takes 5 to 8 months after permits are issued. Attached ADUs are faster at 3 to 5 months of construction.

Is a Passive House ADU worth the extra cost? A Passive House ADU adds 10% to 15% to construction cost but reduces heating and cooling bills by up to 90%. Over 20 years, the energy savings cover the premium. The comfort improvements, including no drafts, even temperatures, and filtered air, are immediate.

Can I build two ADUs on my lot in Seattle? Under Washington's 2023 ADU legislation (HB 1337), cities with populations over 25,000 must allow at least two ADUs per residential lot. Most King County jurisdictions now permit this, though lot size and zoning requirements vary.