TL;DR: ADU costs on the Seattle Eastside in 2026 range from roughly $80K for a simple garage conversion to $650K+ for a premium detached DADU on a complex lot. Detached DADUs in the 600 to 800 sq ft range typically run $280K, $450K all-in. Understanding which cost drivers apply to your specific lot in Kirkland, Bellevue, Redmond, or Sammamish is the single most important step before committing to a project.
What I See Every Week at Piper Cole Architects
Homeowners call me with ADU questions every single week, and the first thing most of them ask is: “How much does an ADU cost?” The honest answer is that it depends on far more variables than most online calculators will admit. After designing ADUs across Kirkland, Bellevue, Redmond, and Sammamish, I can give you real numbers grounded in what Eastside subcontractors are actually charging in 2026, not national averages that have no relationship to our local labor market.
Washington State’s HB 1337, fully in effect, has unlocked tremendous ADU potential across the Eastside. But expanded legal access doesn’t make costs disappear. Let me walk you through what you’re actually looking at.
Estimate your ADU cost
Set the size and finish level below for a planning range on an attached ADU, a detached backyard cottage or a garage conversion. It uses the same Eastside figures as the tables on this page. It is a planning range, not a quote: call David on 425 753 6452 to talk about your lot.
Eastside ADU Cost Estimator
A planning-stage estimate for accessory dwelling units in Seattle & the Eastside, based on Piper Cole Architects’ project data.
Estimate only, not a quote. Ranges reflect design, permitting, and construction for architect-led ADUs on the Seattle Eastside and vary with site conditions, slope, utilities, and finishes. Figures align with our published ADU cost guide. For a fixed scope and fee, contact Piper Cole Architects.
Planning a project?
Wondering what an ADU on your lot would cost?
Send David the address and a few lines about what you want. The first feasibility check is free and usually takes about a day: what the site and the code are likely to allow, and what to check first.
Architect since 1996. Three Masonry Excellence Awards for commercial projects; the Carr residence was featured in Above The Lake and Lux magazines.
ADU Type Cost Table: Eastside 2026
These are all-in project costs, design, permits, construction, and utility connections, for typical lots in Kirkland, Bellevue, Redmond, and Sammamish. Your number may land outside these ranges depending on the site-specific factors covered below.
| ADU Type | Typical Size | All-In Cost Range |
|---|---|---|
| Garage conversion (JADU) | 400 to 600 sq ft | $80,000 to $250,000 |
| Basement ADU | 600 to 900 sq ft | $100,000 to $350,000 |
| Attached addition ADU | 600 to 900 sq ft | $150,000 to $400,000 |
| Detached DADU (standard) | 600 to 800 sq ft | $280,000 to $450,000 |
| Detached DADU (premium/complex) | 800 to 1,000 sq ft | $350,000 to $650,000 |
Garage conversions sit at the lower end because the structure already exists, you’re spending money on interior partition walls, insulation, a bathroom rough-in, electrical upgrade, mechanical, and finishes, not foundations or framing. Premium detached DADUs on hillside lots with challenging utility access and high-end finishes push past $600K.
What Drives Cost Variation on the Eastside
Lot Slope and Geotechnical Requirements
This is the single biggest wildcard I deal with in Kirkland and Sammamish. A flat lot in Redmond’s Overlake neighborhood is a different project than a steeply sloped parcel near Juanita Bay or the Sammamish Plateau. When grade change exceeds roughly 15%, we often need a geotechnical engineer’s report ($3,500 to $7,000) before the city will issue a grading permit. Retaining walls on steep sites can add $30,000 to $80,000 or more depending on height and soil conditions.
Utility Access
Detaching a DADU from the main house requires new or extended utility services. New water and sewer laterals on longer runs, electrical service from the street, and gas line extensions can total $25,000 to $60,000, and that’s before you’ve framed a single wall. On some Sammamish and Redmond parcels that are still on septic, we have to evaluate whether the existing system can handle the added load or whether a new septic system (or connection to public sewer) is required. That decision alone can shift a project budget by $40,000+.
Finish Level
A garage conversion with LVP flooring, builder-grade cabinets, and a standard bathroom runs very differently than one finished with engineered hardwood, custom cabinetry, and a spa-quality bath. I tell clients to budget finish escalation at $30 to $60 per square foot between standard and premium levels.
City Permit Fees
See the city-by-city comparison below. These are not trivial.
Soft Costs: The Budget Line Nobody Budgets For
Soft costs, everything except sticks and nails, typically run 20 to 35% of your total project cost on an Eastside ADU. Here’s how they break down:
Architecture fees:
Percentage-priced firms run roughly 8% to 15% of construction cost, about $17,500 to $52,500 on a $350,000 DADU. Piper Cole quotes a fixed fee instead. Full-service firms handle design through construction administration; drafting-only services cost less but leave you more exposed during construction.
Survey:
$2,000 to $3,500 for a boundary/topographic survey, required on almost every detached ADU project.
Structural engineering:
$2,000 to $4,000 for standard projects; more on complex hillside work.
Geotechnical report:
$3,500 to $7,000 when required by slope or soil conditions.
Permits:
Variable, see city table below.
Utility connections:
$25,000 to $60,000 for new laterals on detached DADUs.
Electrical panel upgrade:
$5,000 to $15,000. Most pre-2000 Eastside homes need this when adding a DADU.
City-by-City Permit Fee Comparison
Permit fees vary significantly across the Eastside. These are approximate 2026 estimates for a 700 sq ft detached DADU at a $300,000 construction valuation:
| City | Estimated Permit Fee | Notes |
|---|---|---|
| Kirkland | $8,000 to $14,000 | MyBuildingPermit.com; review times 8 to 14 weeks |
| Bellevue | $9,000 to $16,000 | Plan review plus fire review if sprinklers required |
| Redmond | $7,500 to $13,000 | ePermits portal; often faster review than Kirkland |
| Sammamish | $6,500 to $11,000 | Competitive fees; critical areas review adds time |
These figures include plan check, building permit, and development impact fees where applicable. They do not include utility connection fees, which are separate and significant.
2026 Cost Escalation on the Eastside
Eastside construction costs are escalating at 4 to 6% annually in 2026, driven by persistent labor shortages in framing, concrete, and mechanical trades, continued materials inflation on lumber and electrical components, and subcontractor capacity constraints from major commercial development in Bellevue and Redmond. If you’re planning an ADU, waiting 12 months to “see if costs come down” is historically a losing bet in this market.
Rental Income ROI: Does an ADU Pencil Out?
For a detached 700 sq ft DADU in Kirkland’s Juanita neighborhood, here’s a real-world example I walked through with a client earlier this year:
Total project cost:
$390,000
Monthly rental income:
$2,600 to $3,100 (2026 Kirkland DADU market rate)
Annual gross income:
$31,200 to $37,200
Annual net income (after expenses, vacancy, mgmt):
~$24,000 to $29,000
Simple payback:
13 to 16 years
Long-term property value increase:
$200,000 to $300,000 at current Kirkland multifamily cap rates
When you factor in property value appreciation, the ADU is often a compelling investment even if the rental income alone looks modest on paper.
The Two-ADU Strategy
HB 1337 now permits most Eastside single-family lots to accommodate two ADUs, one attached and one detached. I’m seeing increasing interest in what I call the “two-ADU strategy” a junior ADU (JADU) converted from an existing garage combined with a new detached DADU in the backyard. Done right, this can generate $4,500 to $6,000/month in combined rental income while leaving the main house intact for the owner’s use.
The sequencing, financing structure, and permitting order on a two-ADU project matter enormously. Our ADU design process page walks through how we typically structure these projects.
Where to Learn More
If you’re evaluating an ADU on the Eastside, these resources are your next steps:
- Understand Washington’s ADU law changes: Washington ADU Laws 2026 Eastside Guide
- ADU design services in Bellevue: ADU Architect Bellevue WA
- ADU design services in Kirkland: ADU Architect Kirkland WA
- Garage conversion specifics: Garage Conversion ADU Seattle Eastside
- Ready to talk numbers on your lot: Contact Us
The Delivery Route Changes How the Price Is Presented
The construction number barely moves between delivery routes, but how it reaches you does. In design-build the design and engineering fee sits inside one quoted number rather than on a separate invoice, which is often misread as design-build being cheaper. It is bundled, not absent. The three ADU delivery routes compared sets out where the money actually sits in each.
Frequently Asked Questions
How much does a detached ADU cost on the Seattle Eastside in 2026?
A detached DADU on the Seattle Eastside (Kirkland, Bellevue, Redmond, Sammamish) typically costs $280,000 to $450,000 all-in for a 600 to 800 sq ft unit, or $350,000 to $650,000 for a premium 800 to 1,000 sq ft unit on a complex lot. Soft costs including architecture, permits, and utility connections typically add 20 to 35% on top of construction cost.
What is the cheapest type of ADU to build on the Eastside?
Garage conversions (JADUs) are typically the least expensive ADU type, ranging from $80,000 to $250,000 all-in, because the existing structure eliminates foundation and framing costs. Basement ADUs can also be cost-effective at $100,000 to $350,000, depending on the extent of structural and egress work required.
How much do permit fees cost for an ADU in Kirkland or Bellevue?
For a typical 700 sq ft detached DADU at a $300,000 construction valuation, Kirkland permit fees run approximately $8,000 to $14,000 and Bellevue fees run $9,000 to $16,000. These figures include plan check and building permit fees but not utility connection fees, which are separate and typically add $25,000 to $60,000 for new service laterals.
Does building an ADU in Kirkland increase my property value?
Yes. A well-designed detached DADU in Kirkland typically adds $200,000 to $300,000 in property value at current Eastside multifamily cap rates, in addition to generating $2,600 to $3,100/month in rental income. The combination of income stream and value appreciation makes ADUs one of the strongest home investment categories in King County.
How long does it take to get an ADU permit on the Eastside?
ADU permit review times on the Seattle Eastside currently range from 8 to 14 weeks in Kirkland, slightly faster in Redmond (often 6 to 10 weeks via their ePermits portal), and 10 to 16 weeks in Bellevue for projects requiring fire review. Total project timeline from design start to occupancy is typically 12 to 18 months for a detached DADU.
Work With Piper Cole Architects
I’m David Meade, AIA, NCARB, principal of Piper Cole Architects in Kirkland, WA. I’ve designed ADUs across Kirkland, Bellevue, Redmond, Sammamish, and the broader Eastside, attached additions, garage conversions, basement units, and detached DADUs on flat lots and challenging hillside sites alike. When you call our office, you talk directly to a licensed architect who will give you a straight answer about what your project is likely to cost and whether it makes sense for your lot.
Contact Piper Cole Architects, or call us at 425-753-6452.
Sources
- Washington State HB 1337 (2023), effective July 23, 2023, statewide ADU provisions
- City of Kirkland Building Division, 2026 Fee Schedule (MyBuildingPermit.com)
- City of Bellevue Development Services, 2026 Permit Fee Schedule
- Seattle-area residential construction cost data, Q1 2026 (Eastside subcontractor surveys)
Authoritative sources
Regulatory requirements referenced in this guide are set by the official agencies below. Rules change, always confirm current requirements with the source:
