Looking to build an ADU this year? Here’s how much a premium garage conversion, detached ADU, and addition ADU takes.
Accessory dwelling units (ADUs) are one of the most talked-about home improvements in high-demand markets like Bellevue Washington. In 2026, the ADU Cost in Bellevue ranges from $180k for a low-end garage conversion to upwards of $600k+ for a luxury detached ADU. Whether you’re looking to create rental income, house family members, or simply add valuable living space, it’s important to understand the cost factors before you start planning.
In this guide, we’ll break down what homeowners should expect to spend on an ADU in Bellevue in 2026, what contributes to the total cost, the types of ADUs you can build, and how to get an accurate estimate for your project.
Jump to a Section:
- What Makes Up The ADU Cost In Bellevue
- Types of ADUs and Typical Costs
- Why ADUs Cost What They Do
- Does an ADU Add Value?
- How NW Build Can Help You Estimate Your ADU Costs
What Makes Up The ADU Cost In Bellevue
Before you go hunting for estimates, here are the key components that go into a Bellevue ADU project Understanding these will help you budget easier and give you a better sense of what your ADU costs in Bellevue:
1. Planning, Architecture & Permits
Before construction begins, design and planning are required including architectural drawings, structural engineering, and city permits. Bellevue has specific ADU zoning and impact fees (e.g., some school impact fees may apply if you’re close), and permit costs are often significant – around $15-28k.
2. Site Preparation & Utility Connections
Contractors will need to clear space, grade, dig foundations, and connect to water, sewer, and power (it’s a mini home after all). If your lot has challenging access, grading issues, or utility tie-in complications, costs can rise quickly. But if you’re unsure, your contractors will find out in an inspection.
3. Construction Costs
This is usually the biggest chunk of your budget – framing, roofing, HVAC, interior walls, plumbing, electrical, windows, and finishes – the actual construction. Costs vary widely depending on whether the unit is:
- Attached to your home (addition ADU)
- Converted from existing space (garage conversion)
- Detached (a new building in your yard)
Detached units generally cost more because they require a new foundation and full utility connections.
4. Interior Finishes
Choices like flooring, cabinets, countertops, lighting, bathroom fixtures, and appliances greatly influence the total cost. High-end finishes can push your project toward the upper range of the budget.
Types of ADUs and Typical Costs
Here’s how costs can vary depending on the type of ADU you choose:
Detached ADU (DADU)
Typical ADU Cost in Bellevue: $450,000–$650,000+
A standalone structure in your backyard. It’s essentially a mini home equipped with a kitchen, bathroom, bedroom, and perhaps even a living room if you choose. This is the most premium version of an ADU as it’s an entire new structure added to your lot that requires specific permits and lot qualifications.
Attached ADU
This is your standard ADU, an addition directly attached to your existing home.
Typical ADU Cost in Bellevue: $300,000 – $500,000+
Usually less expensive than detached since some systems can tie into the original house more easily. The cost can vary due to the effort that’s needed to tie into your existing utilities and structure. And your home might need more structural reinforcement depending on the ADU you wish to build.
Garage ADU Conversion
Transforming your garage into a livable unit.
Typical ADU Cost in Bellevue: $180,000 – $350,000+
Often the least expensive ADU, because much of the structure (roof, walls) is already there. Site work is usually minimal, but there might be zoning/parking requirements you and your contractor will have to look into. But this is the most cost-efficient way to create an ADU and make your garage useful if it isn’t housing cars.
Basement ADU
Converting an existing basement or lower level.
Typical ADU Cost in Bellevue: $220,000 – $400,000+
With basement ADUs, they’ll need egress windows (safety requirement), waterproofing and potential insulation upgrades to ensure it’s compliant with Washington State laws, which is why the cost might be higher.
Note: Costs per square foot can vary depending on site conditions and finishes — typical ranges across Washington are often cited as $300–$500 per square foot or more depending on complexity.
Why Do ADUs Cost So Much?
Several factors specific to Bellevue and the Eastside influence ADU pricing:
- Labor & Material Costs: Construction in this region is more expensive than in many parts of the country.
- Permit & Impact Fees: Bellevue’s permitting process involves plan review, inspection, and sometimes impact fees.
- Site Variables: Soil, slope, existing utilities, and tree protection requirements can add significant costs.
- Higher Standards: Building codes for livable units (kitchens, bathrooms, separate utilities) make ADUs closer to full homes than simple add-ons.
Because Bellevue is part of the greater Seattle/Eastside market, ADU construction costs tend to be higher than the national average — driven by labor rates, strict permitting requirements, and site-specific variables.
And because ADUs are essentially small homes, they involve many of the same systems — foundation, plumbing, electrical, insulation, and finishes — so even relatively compact units aren’t inexpensive when built to code.
Does an ADU Add Value?
While the upfront investment can be substantial, ADUs often deliver solid long-term value. A well-designed ADU can increase your property’s usable space, provide rental income, and contribute to your home’s overall resale value.
On top of that, you can rent out your DADU legally, which is one of the biggest reasons why homeowners in the Bellevue and Seattle area invest in them.
(Learn more about ADUs and DADUs, here.)
How NW Build Can Help You Estimate Your ADU Costs
Because every property, lot, and project vision is unique, the best way to understand what an ADU will cost you in Bellevue is with a personalized estimate.
At NW Build, we’ve specialized in ADU design, permitting, and construction in Bellevue and the surrounding area for over 20 years. We’ll:
- Evaluate your property constraints and opportunities
- Help you choose the right ADU type for your goals
- Provide transparent budgeting and cost projections
- Guide you through permits and construction from start to finish
👉 When you start planning your ADU project, contact NW Build to get your Bellevue ADU cost estimate and start planning your project with confidence.
