If you’ve started pricing out an ADU in King County in 2026, you’ve probably noticed something frustrating: every quote is wildly different. One builder says $180,000. Another says $420,000. A neighbor swears their detached DADU came in at $260,000 turnkey. So what’s the real number?

The honest answer is that ADU cost in 2026 depends on five things — size, foundation, finishes, your lot, and which Eastside city you’re permitting in. At Prolific Design-Build and Restoration, we build accessory dwelling units across Issaquah, Sammamish, Bellevue, Redmond, Kirkland, and Renton, and we’re seeing prices that have stabilized but shifted in interesting ways for 2026. This guide breaks down what homeowners are actually paying right now, why prices vary so much, and how to keep your budget under control without sacrificing the design you want.

The Short Answer: 2026 ADU Cost Ranges in King County

Across the Eastside this spring and summer, we’re seeing these typical all-in costs (design, permits, construction, finishes, utility hookups):

• Attached ADU (interior conversion or addition, 400–800 sq ft): $180,000 to $360,000
• Detached DADU (new structure, 500–1,000 sq ft): $260,000 to $520,000
• Garage conversion ADU (existing 400–600 sq ft garage): $95,000 to $185,000
• Basement conversion ADU (400–800 sq ft): $85,000 to $170,000

On a per-square-foot basis, expect $300 to $550 for new detached construction and $180 to $320 for conversions of existing space. Premium finishes, complex sites, or hillside lots in places like Cougar Mountain or Bridle Trails can push numbers well above these ranges.

Why Detached DADUs Cost So Much More Than Conversions

The single biggest cost variable is whether you’re building new from the ground up or converting space you already have. A detached DADU is essentially a small house — it needs its own foundation, framing, roof, exterior envelope, full mechanicals, utility connections, and a separate water meter or sewer connection in many King County jurisdictions. That’s why even a modest 600-square-foot DADU rarely comes in under $280,000 in 2026.

A garage conversion, by contrast, already has a slab, walls, and a roof. You’re adding insulation, plumbing, a kitchen, a bathroom, electrical upgrades, and finishes — significant work, but a fraction of building new. Basement conversions follow similar logic: the shell is paid for, and you’re upgrading what’s already there to habitable standards.

For homeowners on a tighter budget who still want the flexibility an ADU provides, conversions remain the clear winner in 2026. We’ve completed multiple garage-conversion ADUs in Renton and Sammamish for under $150,000 that feel just as livable as new construction.

The Major Cost Drivers (Beyond Square Footage)

Size matters, but four other factors swing budgets just as hard:

1. Site Conditions

Flat, accessible lots with utilities at the street are the cheapest to build on. Sloped lots (common throughout the Issaquah Highlands, Cougar Mountain, and the Sammamish plateau) require retaining walls, deeper foundations, and sometimes engineered drainage — easily adding $25,000 to $75,000. If your lot has mature trees that need protection or removal, septic instead of sewer, or rocky soil, plan for more.

2. Utility Connections and Impact Fees

Connecting a new detached DADU to water, sewer, gas, and electric is one of the biggest line items homeowners underestimate. Depending on your city, expect $15,000 to $40,000 for utility extensions and impact fees alone. Bellevue and Redmond tend to run higher than Renton and unincorporated King County.

3. Foundation Type

A simple slab-on-grade foundation is the most affordable. Crawl space foundations add $8,000–$15,000. Full basements under a DADU can add $40,000+ but also add significant usable square footage.

4. Finishes and Fixtures

This is where homeowners have the most control. A builder-grade finish package (LVP flooring, quartz counters, stock cabinetry) keeps costs reasonable. Stepping up to designer-grade materials — real wood, stone counters, custom cabinetry, integrated appliances, and the warm tones, curves, and concealed storage trending in 2026 — can add $30,000 to $90,000 to even a small ADU.

City-by-City Permit Costs in King County

Permit and review fees vary noticeably across Eastside cities. Here’s what we’re seeing in 2026:

• Issaquah: Building permit, plan review, and impact fees typically run $9,000–$18,000 for a new DADU. The city has actively streamlined its ADU process and offers a pre-approved plan program that can shave weeks off review time.
• Bellevue: Permits, school impact fees, transportation impact, and utility connection fees often total $14,000–$28,000. Bellevue’s review process is rigorous but predictable when plans are complete.
• Sammamish: Expect $10,000–$20,000 in fees. Critical area reviews are common given the wetlands and slopes throughout the city.
• Redmond: Roughly $11,000–$22,000 in combined fees. Redmond’s ADU pre-approved plan program is one of the strongest on the Eastside.
• Kirkland: $10,000–$20,000 typical. Kirkland recently expanded ADU eligibility in single-family zones.
• Renton: $8,000–$16,000 in fees, often the most affordable Eastside city for permitting a DADU.

If you’re trying to maximize what your dollar buys, Renton and unincorporated King County typically offer the most budget-friendly permitting environment. For deeper detail, see our city-specific permit guides for Issaquah and Bellevue.

How 2026 Design Trends Are Shaping ADU Pricing

The 2026 design shift away from sterile gray-and-white minimalism toward warm tones, curves, natural materials, and concealed storage is showing up in ADU projects too. Clients are asking for arched doorways, warm oak cabinetry, plaster or limewash walls, and built-in storage that hides clutter — exactly the design language we’re using in larger King County kitchen and bathroom remodels.

These trends don’t dramatically change base construction cost, but they do raise finish budgets. A 700-square-foot detached DADU with rich warm-toned finishes, real wood flooring, custom millwork, and a curved-arch entry will typically run $50,000–$80,000 more than the same footprint with builder-grade materials. For many homeowners the upgrade is worth it because an ADU often doubles as a long-term rental, guest suite, or aging-in-place dwelling — and tenants and family members notice quality.

The ROI Question: Is an ADU Worth It in 2026?

Three real-world payoffs are driving the Eastside’s ADU boom:

Rental income. A well-finished 600–800 sq ft DADU in Bellevue, Kirkland, or Redmond rents for $2,200–$3,200/month in 2026. Even on the lower end of that range, a $300,000 ADU build can return $26,000+ per year in gross rent, with appraised value typically increasing $150,000–$250,000.

Multigenerational living. Aging parents, adult children, or family caregivers gain independence and privacy without the cost of separate housing. Our multigenerational living design guide covers the layout choices that make this work.

Long-term flexibility. An ADU is one of the most flexible square-footage investments available — home office today, rental next year, in-law suite after that, or even a primary residence when the main house is downsized.

From an energy and operating-cost perspective, building ADUs to current code (heat pumps, heat-pump water heaters, well-sealed envelopes) keeps utility bills low. See our energy-efficient remodel ROI guide for the latest 2026 payback numbers.

Financing an ADU in King County

Most King County homeowners finance ADUs in one of four ways: home equity loans, HELOCs, cash-out refinance, or construction-to-permanent loans through credit unions and regional banks. Washington’s recently expanded ADU lending products allow some borrowers to count projected rental income toward qualifying — a meaningful change since 2024.

We always encourage clients to talk to two or three lenders before committing. A small difference in rate or loan structure on a $300,000 project can shift your real cost by tens of thousands over the life of the loan.

How to Save Money Without Cutting Corners

Smart cost control on an ADU rarely means buying cheaper materials. It means making smarter decisions earlier:

• Right-size the build. A 600 sq ft DADU isn’t dramatically less functional than 900 sq ft, but the cost gap is often $80,000–$150,000.
• Use a pre-approved plan. Issaquah, Redmond, and Seattle have pre-approved DADU plans that drop architecture costs and shorten review.
• Pick a simple roofline. Gables and shed roofs framed efficiently can save $10,000–$25,000 versus complex hip or dormer designs.
• Bundle projects. If you’re already remodeling a kitchen or bath, sharing one mobilization and crew across multiple projects saves real money. See our breakdown of project bundling savings.
• Convert first, build later. A garage or basement conversion can deliver 70% of the ADU benefit for 35% of the cost.
• Pick the right contractor. A design-build firm that handles design, permitting, and construction in-house typically saves 8–15% over hiring an architect, a permit expediter, and a general contractor separately.

Realistic Timeline for an ADU in 2026

From the day you sign a design agreement to the day you receive a certificate of occupancy, plan on 9–14 months for a new detached DADU and 5–8 months for a conversion. Roughly:

• Design and engineering: 6–10 weeks
• Permit review (varies by city): 8–18 weeks
• Construction (DADU): 4–7 months
• Construction (conversion): 2–4 months
• Inspections and certificate of occupancy: 2–4 weeks

Starting design work in spring or early summer 2026 typically positions you to complete construction by late spring or summer 2027 — ideal timing for an Eastside rental.

Why Design-Build Is the Right Fit for ADU Projects

ADUs are uniquely well-suited to a design-build approach because design choices and budget have to stay tightly connected. With separate architects, expediters, and contractors, scope creep is common and budgets balloon. Under one design-build roof, every design decision is priced in real time. You see the cost impact of an arched ceiling, an upgraded kitchen island, or a heat-pump water heater before you sign off on it.

If you’re considering an ADU and want a real number for your specific lot, your specific city, and your specific finishes, the next step is a site visit. We’ll walk the property, talk through your goals (rental, family, flexibility), and put together a transparent budget with no surprises.

Get a Real ADU Cost Estimate for Your Property

Prolific Design-Build and Restoration is a licensed and insured contractor serving Issaquah, Sammamish, Bellevue, Redmond, Kirkland, Renton, and the rest of King County. We are proudly Black-owned and Latino-owned, and we build accessory dwelling units, additions, and full remodels with the same craftsmanship and accountability we’d expect on our own homes.

Call (425) 800-4775 or request a free consultation and we’ll give you a realistic 2026 ADU cost range for your property — not a generic ballpark, but a number based on your lot, your city, and the design you actually want.

Related: ADU & DADU Construction in Issaquah, Sammamish & Bellevue · Garage Conversions in King County: Costs and Permits · Multigenerational Living Design