How Much Does An ADU Cost In Bellevue In 2026?

If you are weighing an accessory dwelling unit (ADU) for your Bellevue property in 2026, the first question is almost always the same: what will it actually cost? The honest answer is that it depends — on the type of unit, the condition of your site, the finishes you choose and the permitting path your lot requires. As a rough range, plan on anywhere from about $180,000 for a modest garage conversion to $600,000 or more for a large, high-spec detached ADU.

Below we break down what drives those numbers, what each type of ADU typically costs on the Eastside, and how NW Build helps you land on a realistic budget before you commit.

What makes up the cost of an ADU in Bellevue

An ADU budget is really four budgets stacked together. Understanding each one makes the total far less mysterious.

Planning, architecture & permits

Before a shovel hits the ground, you are paying for design, engineering and the City of Bellevue permitting process. Architectural drawings, structural engineering, a site survey and permit fees commonly land between $15,000 and $40,000, depending on complexity and whether you need variances.

Site preparation & utility connections

Every lot is different. Grading, drainage, and running water, sewer and power to a detached unit can add meaningfully to the bill — sometimes $20,000 to $60,000 on a difficult site. A garage conversion that reuses existing connections avoids much of this.

Construction costs

The shell itself — foundation, framing, roofing, windows, mechanical, electrical and plumbing — is the largest single line item, and it scales directly with square footage.

Interior finishes

Cabinetry, flooring, tile, countertops and fixtures are where budgets swing the most. The same footprint can be finished modestly or to a luxury standard, and that choice alone can move the total by six figures.

Types of ADUs and their typical costs

Here is what the major ADU types generally run in the Bellevue area in 2026. Treat these as planning ranges, not quotes.

• Detached ADU (DADU): a standalone unit in your yard — the most flexible and the most expensive, typically $450,000–$650,000+.
• Attached ADU: an addition sharing a wall with the main house, usually $300,000–$500,000+.
• Garage conversion: reworking an existing garage into living space, generally $180,000–$350,000+.
• Basement ADU: finishing and legalizing a lower level, commonly $220,000–$400,000+.

A detached ADU costs the most because you are building a small home from the ground up — its own foundation, roof and utilities. A conversion is cheaper largely because that structure already exists.

Why do ADUs cost so much?

ADUs carry the same fixed costs as a full house — permits, a kitchen, a bathroom, HVAC, code-compliant electrical and plumbing — but spread over far fewer square feet, so the cost per square foot runs high. Add the Puget Sound region’s labor and materials pricing, and premium finishes, and it is easy to see why a small building carries a large number.

Does an ADU add value?

For most Bellevue homeowners, yes. A well-built ADU can generate rental income, house family members, and increase the resale value of the property. On the Eastside, where land is scarce and demand is strong, a permitted, professionally built ADU is one of the more reliable ways to add usable square footage and long-term value to a lot.

How NW Build can help you estimate your ADU costs

Every number above is a starting point — your real budget depends on your lot, your goals and your finishes. NW Build has been designing and building across the Puget Sound region since 2006, and we walk every ADU client through a clear, itemized estimate before any commitment, so there are no surprises once the build begins.

If you are considering an ADU in Bellevue or anywhere on the Eastside, request a consultation and we will help you understand exactly what your project will take.