Introduction
If you are a Seattle area homeowner considering an accessory dwelling unit for family, rental income, or extra living space, you probably have one big question: how much will it cost?
In this guide from A-Z Construction, we walk you through realistic cost ranges, key cost drivers, permitting considerations, and area by area estimates around Seattle. Our goal is to help you budget smart and avoid surprises.
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What Is an ADU in Seattle and King County?
An ADU is a self contained living space on the property of an existing home. Common forms in our region include detached ADUs, attached ADUs, and interior ADUs. Because ADUs come in many forms, costs can vary widely.
Seattle and Eastside ADU Costs by Type in 2026
Below are ballpark ranges based on recent projects, local contractor bids, and current permit and regulatory realities. Use them as a starting point, not a guarantee.
Garage or Accessory Conversion, Attached
- Typical size: 400 to 800 square feet
- Estimated cost: 200,000 to 350,000 dollars
- Notes: makes use of an existing structure, but often needs upgrades to foundation, insulation, plumbing, and electrical
Interior Conversion, Basement or Upper Floor
- Typical size: 500 to 1,000 square feet
- Estimated cost: 275,000 to 475,000 dollars
- Notes: may involve excavation, structural reinforcement, separate egress, and utility tie ins
Detached ADU, Basic
- Typical size: 600 to 1,000 square feet
- Estimated cost: 350,000 to 600,000 dollars
- Notes: standard finishes and relatively simple site conditions
Detached ADU, Mid to High End
- Typical size: 800 to 1,000 square feet
- Estimated cost: 500,000 dollars and up
- Notes: premium materials, custom architecture, more complex sites, and higher performance systems
These ranges generally include design, permitting, site work, structure, utilities, and interior finishes. They do not include extreme site challenges such as steep slopes, rock, or major tree removal.
Smaller ADUs often have a higher effective cost per square foot because fixed costs like permits, design, and utility hookups are spread over less area. Ultimately, the final cost of an ADU comes down to a variety of factors that our team at A-Z can walk you through to make your budget serve you best.
What Influences ADU Costs in Seattle and King County
When budgeting, consider these major factors:
- Site conditions and access: Soils, slope, drainage, and tree protection can add foundation and earthwork costs.
- Utility tie ins: Extending water, sewer, storm, electric, and gas can be significant if trenching through hardscape or difficult terrain.
- Permitting and regulatory costs: Expect plan review, inspections, and utility capacity charges. Architectural, structural, geotech, and survey services may add to soft costs, especially for custom designs.
- Design complexity and finish level: Simple forms and standard finishes cost less. Custom layouts, higher ceilings, premium cabinetry, and luxury appliances add to the budget.
- Labor and schedule premiums: Regional labor rates and busy season timelines may increase pricing.
- Energy code and mechanical systems: Seattle’s energy and mechanical requirements can raise engineering and system costs, especially for high performance builds.
- Permit delays and plan revisions: Incomplete or nonconforming plans lead to resubmittals. Pre approved DADU plans can reduce review time and design cost.
Cost per Square Foot and Why It Can Mislead
Many homeowners look for a single cost per square foot, but ADUs always require fixed elements such as kitchens, bathrooms, plumbing, and permits. That means a 400 square foot ADU might land near 300,000 dollars, while a 1,000 square foot ADU might be 500,000 dollars. The smaller project shows a higher cost per square foot because fixed costs are spread over fewer square feet. Always ask what is included before trusting a per square foot figure.
ADU Pricing by Area in Seattle and the Eastside
While basement remodel prices in the Seattle area follow similar trends, local factors can make a difference in your overall budget. Below are some neighborhoods and towns where homeowners frequently undertake basement remodels. We’ve provided estimated cost ranges for each area, followed by a list of additional communities in or near Seattle.
Seattle ADU Costs
In-city ADUs are especially popular for rental income, guest suites, and multigenerational living. With tighter lots, sloped terrain, and detailed code requirements, most detached ADUs in Seattle range from 350,000 to 650,000 dollars or more, while basement or attached conversions often fall between 250,000 and 450,000 dollars.
Neighborhoods like Ballard, Greenwood, West Seattle, and Capitol Hill often feature compact, efficient ADUs that make the most of smaller urban lots. Many are built as rental cottages or home offices designed to complement Seattle’s classic craftsman and mid-century architecture.
Bellevue ADU Costs
Bellevue homeowners often build ADUs to support multigenerational living or as high-end guesthouses. With large properties, luxury homes, and strong design review standards, detached ADUs in Bellevue usually start around 375,000 dollars and can exceed 700,000 dollars.
In areas like Somerset, Enatai, and Bridle Trails, detached ADUs are commonly built with the same architectural detail as the main home. Many include full kitchens, private patios, and premium finishes for visiting family or live-in caregivers.
Mercer Island ADU Costs
Mercer Island’s ADUs are some of the most refined in the region. Homeowners here typically build guesthouses, poolside suites, or detached in-law cottages to match the upscale look of their main residence. Because of larger lot sizes and higher-end design expectations, most ADUs range from 400,000 to 750,000 dollars and up.
These projects often emphasize privacy and craftsmanship. Many include high ceilings, floor-to-ceiling windows, luxury bathrooms, and integrated outdoor living areas with views of Lake Washington. Homeowners here tend to prioritize longevity, comfort, and architectural harmony over minimalist, budget builds.
Issaquah ADU Costs
Nestled near the base of the Cascade foothills, Issaquah’s sloped sites and wooded lots often add engineering and grading costs. ADUs typically range from 340,000 to 600,000 dollars, depending on site access and foundation needs.
Homeowners in Issaquah frequently choose basement conversions or attached ADUs to create flexible space for family or guests. Detached ADUs are also gaining popularity in neighborhoods with larger lots, offering quiet retreats or potential rental income close to hiking trails and downtown amenities.
Sammamish ADU Costs
Sammamish homeowners love ADUs for family living flexibility and rental potential. The area’s rolling terrain, tree protection requirements, and hillside lots make site work a major cost driver. Most detached ADUs fall between 350,000 and 650,000 dollars.
Many Sammamish ADUs are designed as modern cottages, private studios, or garden suites for adult children and extended family. Some homeowners also include workspace or gym elements to maximize year-round use.
Woodinville ADU Costs
Known for its vineyards and semi-rural charm, Woodinville offers larger lots that simplify detached ADU construction. Costs are generally lower than in Bellevue or Seattle, with most projects ranging from 320,000 to 550,000 dollars.
Homeowners here often build standalone backyard cottages or studio spaces to host guests, run home-based businesses, or generate Airbnb-style rental income. Detached designs with rustic-modern finishes that blend with the wooded surroundings are especially popular.
Renton ADU Costs
Renton’s mix of newer subdivisions and established neighborhoods makes it ideal for both garage conversions and new detached ADUs. With relatively easier site access and more flexible zoning in many areas, ADU costs range from 320,000 to 575,000 dollars.
Homeowners here often use ADUs as long-term rental units, guest suites, or private workspaces. Detached units are especially common in areas like Fairwood and Talbot Hill, where lot sizes allow for comfortable placement without compromising outdoor living space.
Newcastle ADU Costs
Newcastle combines suburban convenience with wooded hillside settings. Many ADUs here are attached additions or detached cottages used for family members or guests.
Because of mixed terrain, pricing generally falls between 330,000 and 600,000 dollars. Larger lots and quiet neighborhoods make detached ADUs a popular way to expand living space without altering the main home’s layout. Homeowners often invest in custom details to match their existing architecture and landscaping.
Redmond ADU Costs
Redmond’s combination of newer suburban developments and wooded acreage creates a broad range of ADU styles. Costs typically range from 350,000 to 600,000 dollars for detached units, depending on design and lot conditions.
Homeowners in neighborhoods like Education Hill and Overlake often build in-law suites, backyard rentals, or guest cottages. Many ADUs serve as independent living spaces for family members working in the nearby tech sector, while others provide flexible work-from-home studios or rental units close to Microsoft and Redmond Town Center.
Kirkland ADU Costs
Kirkland offers one of the best balances of urban access and neighborhood feel. With demand high for both rental ADUs and home office cottages, pricing typically ranges from 360,000 to 650,000 dollars.
Areas like Norkirk, Juanita, and Houghton are popular for detached ADUs that feature modern architecture, rooftop decks, and large windows that frame Lake Washington views. Homeowners often choose to design spaces that serve dual purposes — a guest suite today and a rental or workspace tomorrow.
Permit and Regulatory Considerations in Seattle
- Seattle Department of Construction and Inspections manages ADU permits, with fees paid in stages.
- Permit fees adjust annually. Plan for review, inspections, and utility capacity charges.
- ADUs generally do not require off street parking, but if you remove required parking you must replace it.
- Many lots now allow two ADUs, often one attached and one detached.
- Pre approved DADU plans can shorten design time and reduce revision cycles.
Working with an ADU experienced contractor or architect who knows SDCI will save time and reduce surprises.
Why Work with A-Z Construction?
At A-Z Construction, we specialize in ADUs across Seattle and the Eastside. When you partner with us, you get local expertise, transparent budgeting, end to end coordination, and craftsmanship that stands up to Pacific Northwest conditions.
Every A-Z project begins with a site walk, zoning and feasibility review, and a design that fits your property and vision.
Getting Started, An ADU Planning Checklist
- Check zoning and feasibility for your lot size, zone, setbacks, and ADU eligibility.
- Schedule a site walk and survey to review slope, trees, access, and utilities.
- Choose between a pre approved plan or a custom design that fits your site.
- Budget for permits, utility charges, and potential infrastructure upgrades.
- Request a detailed estimate that breaks out site, structure, utilities, and finishes.
- Add a contingency buffer of 10 to 15 percent for unforeseen items.
- Plan for timing, many projects run four to eight months or more from application to final inspection.
Final Thoughts
An ADU in Seattle or the Eastside adds flexibility, function, and value. The best way to set a realistic budget is to start with a site specific feasibility review and a clear scope.
A-Z Construction is ready to walk your property, talk through options, and build a detailed plan. Whether you want a rental, a guest suite, or space for family, we will help you design and build an ADU that feels right at home.
Ready to get started? Contact A Z Construction today to schedule your consultation and begin planning your Seattle ADU.
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