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# How Much Does an ADU Cost in Seattle in 2026? Budgets, Timelines, and What to Know
Building an accessory dwelling unit in Seattle is one of the most common projects King County homeowners are considering right now. Whether the goal is rental income, housing for family members, or adding long-term value to your property, ADUs have become a practical option thanks to Seattle's progressive zoning changes and strong rental demand. Love Construction, a Passivehaus-certified builder based in SeaTac, has built ADUs and DADUs across the greater Seattle area — here is a realistic breakdown of what they cost, how long they take, and what to plan for in 2026.
ADU vs. DADU: Understanding the Two Types
Seattle zoning allows two types of accessory dwelling units on most single-family lots:
Attached ADU (AADU): Built within or attached to the existing primary residence. This could be a basement conversion, an attic buildout, or an addition attached to the main house. Typically the more affordable option because it shares the existing structure's foundation, roof, and utilities.
Detached ADU (DADU): A standalone structure built in the backyard, completely separate from the primary residence with its own foundation, walls, roof, and utility connections. Sometimes called a backyard cottage. More expensive to build but offers greater privacy and rental appeal.
Since 2019, Seattle has allowed homeowners to build both an AADU and a DADU on the same lot, meaning a single-family property can legally contain up to three dwelling units.
ADU Cost Ranges in Seattle (2026)
Construction costs in King County have stabilized somewhat compared to the rapid increases of 2022-2024, but Seattle remains one of the higher-cost markets in the Pacific Northwest. Here are realistic cost ranges based on current Seattle-area pricing:
| ADU Type | Size Range | Cost Range | Cost Per Sq Ft | |----------|-----------|------------|----------------| | Attached ADU (basement conversion) | 400 - 800 sq ft | $100,000 - $200,000 | $200 - $300 | | Attached ADU (addition) | 500 - 1,000 sq ft | $150,000 - $280,000 | $250 - $350 | | Detached ADU (DADU) | 400 - 1,000 sq ft | $180,000 - $350,000 | $300 - $400 | | Passive House DADU | 400 - 1,000 sq ft | $220,000 - $400,000 | $350 - $450 |
These ranges include design, permitting, construction, and standard finishes. They do not include furniture, landscaping, or specialty upgrades beyond standard fixtures and appliances.
What Drives Cost Up
Several factors push ADU costs toward the higher end in Seattle:
- • Site conditions: Sloped lots, limited access for equipment, or poor soil conditions requiring engineered foundations
- • Utility connections: Running new sewer, water, and electrical lines from the street to a detached structure — especially on larger lots
- • High-performance building: Passive House or net-zero energy standards add 15-25% to construction costs but reduce long-term energy expenses to near zero
- • Finishes and fixtures: Custom cabinetry, high-end appliances, and premium flooring can add $20,000-$50,000
- • Permitting complexity: Some lots in Seattle have environmentally critical areas (ECAs), setback challenges, or height restrictions that require variance requests
What Keeps Cost Down
- • Simple rectangular footprints — complex shapes add framing and finishing cost
- • Standard ceiling heights — 8-foot ceilings instead of 9 or 10 foot
- • Mid-range finishes — quality materials that are not custom or imported
- • Flat, accessible lots — easy equipment access and straightforward foundations
- • Existing utility proximity — shorter runs from the street to the ADU location
ADU Construction Timeline in Seattle
From initial consultation to move-in, a typical ADU project in Seattle takes 10 to 16 months. Here is the breakdown:
Design Phase: 2-4 Months
Working with a design-build firm like Love Construction means the design and construction teams are under one roof. Aaron Hundtofte personally oversees both design and build, which eliminates the back-and-forth that happens when homeowners hire separate architects and contractors. During this phase, the floor plan, exterior design, and structural details are finalized.Permitting: 2-4 Months
Seattle Department of Construction and Inspections (SDCI) reviews all ADU permit applications. Current review times fluctuate, but 8-16 weeks is typical for a standard ADU application without variances. A design-build firm familiar with SDCI requirements can prepare a complete submittal that avoids revision cycles.Construction: 4-8 Months
Actual construction time depends on size, complexity, and weather. A straightforward 600 sq ft DADU on a flat lot with standard finishes typically takes 4-5 months of active construction. Larger units, Passive House builds, or challenging site conditions push toward 6-8 months.Total: 10-16 Months
Plan for the full range. Projects with clean permitting and simple sites can come in under a year. Complex builds with Passive House certification and challenging sites may extend beyond 16 months.ADU Rental Income Potential in Seattle
The rental income potential is one of the primary reasons Seattle homeowners build ADUs. Current market rents for ADUs in King County:
| ADU Size | Neighborhood Type | Monthly Rent Range | |----------|-------------------|-------------------| | Studio / 400 sq ft | Central Seattle (Capitol Hill, Ballard, Fremont) | $1,400 - $1,800 | | 1-Bedroom / 550-650 sq ft | Central Seattle | $1,800 - $2,400 | | 1-Bedroom / 550-650 sq ft | South King County (SeaTac, Tukwila) | $1,500 - $2,000 | | 2-Bedroom / 800-1,000 sq ft | Central Seattle | $2,200 - $3,000 | | 2-Bedroom / 800-1,000 sq ft | South King County | $1,800 - $2,500 |
At a $200,000 build cost and $1,800 per month in rent, you are looking at a roughly 9-year payback period before accounting for financing costs, property management, maintenance, and taxes. A $350,000 DADU renting at $2,500 per month follows a similar timeline. These are strong returns for a real estate investment that also increases the assessed value of your property.
Why Passive House ADUs Are Worth Considering
Love Construction is one of the few builders in King County that is Passivehaus certified. A Passive House ADU costs more to build — roughly 15-25% above conventional construction — but delivers measurable advantages:
- • Up to 90% reduction in heating and cooling costs — in Seattle's mild but damp climate, a well-insulated, airtight ADU with heat recovery ventilation can cost almost nothing to heat
- • Superior comfort — consistent temperatures in every room, no drafts, no cold spots
- • Excellent indoor air quality — continuous filtered fresh air through the HRV system, which is especially valuable during wildfire smoke season in the Pacific Northwest
- • Durability — the airtight building envelope resists moisture intrusion, which is critical in Seattle's rain-heavy climate
- • Tenant appeal — renters increasingly value energy efficiency and comfort, and a Passive House ADU commands premium rents
Seattle ADU Zoning Rules You Should Know
Seattle's zoning code has become increasingly ADU-friendly. Key rules as of 2026:
- • No owner-occupancy requirement — you do not have to live on the property to build or rent an ADU
- • Both AADU and DADU allowed on most single-family lots (up to 3 units per lot)
- • No off-street parking required for ADUs
- • DADU maximum size: 1,000 sq ft (in most zones)
- • Height limit: Generally 22-25 feet for a DADU, depending on the zone
- • Setbacks: Typically 5 feet from side and rear lot lines for a DADU
- • Impact fees have been waived for ADUs in Seattle through recent legislation
How to Choose an ADU Builder in Seattle
Not all general contractors have ADU experience. Here is what to look for:
1. Verify the WA contractor license — check the Department of Labor and Industries website (lni.wa.gov) for the contractor's license status, bond, and insurance 2. Ask about ADU-specific experience — how many ADUs have they completed? Can they provide references and photos? 3. Design-build vs. separate contracts — a design-build firm handles architecture, engineering, and construction under one contract, which reduces miscommunication and change orders 4. Permitting knowledge — an experienced ADU builder knows SDCI's requirements and can navigate the permitting process efficiently 5. Energy performance options — ask whether they offer Passive House or high-performance building options, even if you do not choose them initially
Love Construction operates as a design-build firm with Passivehaus certification, WA contractor license LOVECC*802N4, and Built Green certification. Aaron Hundtofte personally oversees every project from design through completion.
Frequently Asked Questions
How much does a basic ADU cost in Seattle?
A basic attached ADU (basement conversion) starts around $100,000. A detached ADU (backyard cottage) starts around $180,000. Final cost depends on size, site conditions, finishes, and energy performance standards. See the cost table above for detailed ranges.
Do I need to live on the property to build an ADU in Seattle?
No. Seattle removed the owner-occupancy requirement. You can build an ADU on a property you own regardless of whether you live there.
How long does it take to get an ADU permit in Seattle?
SDCI permit review currently takes 8-16 weeks for a standard ADU application. Applications requiring variances or environmental review take longer. A complete, well-prepared submittal reduces the risk of revision cycles.
Can I build a Passive House ADU?
Yes. Love Construction is Passivehaus certified and has built high-performance ADUs in the Seattle area. Passive House ADUs cost approximately 15-25% more than conventional construction but deliver up to 90% reduction in energy costs and superior comfort year-round.
Will an ADU increase my property value?
Yes. ADUs in King County typically add significant value to the property — often more than the construction cost, especially in neighborhoods with strong rental demand. The exact value depends on location, size, and quality of construction.
Start Your ADU Project
Love Construction offers a free Passive House feasibility consultation for King County homeowners considering an ADU. This is a no-obligation conversation with Aaron to discuss your project goals, assess your lot's potential, and get a realistic budget range and timeline.
Call (206) 604-5504 or visit loveconstructionseattle.com/contact to schedule your consultation.
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