“ADU Cost Guide: Seattle Eastside 2026

*By David Meade, AIA, NCARB | Piper Cole Architects*

> 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 600–800 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.

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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.

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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)	400–600 sq ft	$80,000–$250,000
Basement ADU	600–900 sq ft	$100,000–$350,000
Attached addition ADU	600–900 sq ft	$150,000–$400,000
Detached DADU (standard)	600–800 sq ft	$280,000–$450,000
Detached DADU (premium/complex)	800–1,000 sq ft	$350,000–$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.

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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–$7,000) before the city will issue a grading permit. Retaining walls on steep sites can add $30,000–$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–$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–$60 per square foot between standard and premium levels.

City Permit Fees

See the city-by-city comparison below. These are not trivial.

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Soft Costs: The Budget Line Nobody Budgets For

Soft costs — everything except sticks and nails — typically run 20–35% of your total project cost on an Eastside ADU. Here’s how they break down:

• Architecture fees: 5–15% of construction cost. On a $350,000 DADU, expect $17,500–$52,500. Full-service firms (like ours) handle design through construction administration; drafting-only services cost less but leave you more exposed during construction.
• Survey: $2,000–$3,500 for a boundary/topographic survey, required on almost every detached ADU project.
• Structural engineering: $2,000–$4,000 for standard projects; more on complex hillside work.
• Geotechnical report: $3,500–$7,000 when required by slope or soil conditions.
• Permits: Variable — see city table below.
• Utility connections: $25,000–$60,000 for new laterals on detached DADUs.
• Electrical panel upgrade: $5,000–$15,000. Most pre-2000 Eastside homes need this when adding a DADU.

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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–$14,000	MyBuildingPermit.com; review times 8–14 weeks
Bellevue	$9,000–$16,000	Plan review plus fire review if sprinklers required
Redmond	$7,500–$13,000	ePermits portal; often faster review than Kirkland
Sammamish	$6,500–$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.

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2026 Cost Escalation on the Eastside

Eastside construction costs are escalating at 4–6% 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.

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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–$3,100 (2026 Kirkland DADU market rate)
• Annual gross income: $31,200–$37,200
• Annual net income (after expenses, vacancy, mgmt): ~$24,000–$29,000
• Simple payback: 13–16 years
• Long-term property value increase: $200,000–$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.

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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–$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.

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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

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Frequently Asked Questions

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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.

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Sources

1. Washington State HB 1337 (2023), effective July 23, 2023 — statewide ADU provisions
2. City of Kirkland Building Division, 2026 Fee Schedule (MyBuildingPermit.com)
3. City of Bellevue Development Services, 2026 Permit Fee Schedule
4. Seattle-area residential construction cost data, Q1 2026 (Eastside subcontractor surveys)