How to Choose an ADU Builder in Snohomish and King County

Searches for ADU builders in Washington have jumped this year, and so has the number of companies offering to build one. An ADU is a small house with all of a big house’s systems and a permit process the big house never had, which makes the choice of builder the biggest decision in the project. Here is how we would choose, if we were the homeowner.

1. Verify the license before the first meeting

Every contractor in Washington must be registered with the Department of Labor & Industries, bonded and insured. Look the company up on L&I’s contractor lookup by name or license number and check that the registration is active, the bond is current, and there are no unresolved claims. An ADU is a six-figure project; the license check takes two minutes. Our guide to choosing a contractor in Washington covers the rest of the paperwork.

2. Ask for ADUs, not just remodels

A kitchen remodeler and an ADU builder share a trade and not much else. ADU work is foundation, utilities, site drainage, fire separation from the main house, and a permit set with civil and structural sheets. Ask for three ADUs the company finished in your county in the last two years, with addresses or owners you can call. Then ask which kind: a detached backyard cottage, a daylight-basement conversion and a garage conversion are different projects, and you want the one that matches yours.

3. Insist on feasibility before design

The expensive ADU mistakes happen before anyone picks up a hammer. A good builder’s first step is a feasibility read on your specific parcel: zoning and lot coverage, setbacks and height, whether a second unit is allowed (under state law it almost always is now, but local size limits vary), where the sewer and water actually run, whether the electrical service can carry a second kitchen, and how a crew and a concrete truck get to the back yard. If a company wants a design deposit before that read, keep looking. Start with what an ADU is under Washington law and what HB 1110 changed so you can tell whether the builder knows the current rules.

4. Understand how the number is built

ADU pricing is where homeowners get hurt. Our ADU cost guide for Snohomish and King County gives the ranges: roughly $40,000 to $150,000 for a garage conversion, $100,000 to $250,000 for a basement unit, and $250,000 to $450,000 for a detached new build in 2026. A proposal well below those before a site visit is usually the shell without the utilities, the permits, the design or the connection fees, each of which can be five figures. Ask what is excluded, in writing. Then read our guide to fixed-price vs cost-plus contracts: an uncapped cost-plus ADU is the most common way a $300,000 project becomes $400,000.

5. One person owns the permit

ADU permits touch building, planning, utilities and often a separate electrical permit, and in Seattle the SDCI review can run months. Someone has to prepare the set, submit, answer correction rounds quickly and schedule inspections, and it should be the builder, not you and not a designer who disappears after the drawings. Ask who files, who responds to corrections, and how the build schedule accounts for the review. Our permit timeline guide shows how long each city actually takes.

6. The questions that sort builders fast

• Who will be on site every day, and have they built an ADU before?
• What did your last three ADUs cost, and what changed from the original estimate?
• How do you handle the sewer connection when the existing line runs under where the ADU goes?
• What is the fire-separation detail between the ADU and the house, or the setback to the property line?
• Does your price include design, engineering, permit fees, utility connection fees and a separate electrical meter if I want one?
• Prefab or site-built, and why for my lot? (Our prefab vs site-built comparison explains when each wins.)

The twelve questions we suggest for any contractor still apply; these six are the ADU-specific ones.

7. Red flags

• A price before anyone has stood in your yard.
• No feasibility step, or no mention of utilities at all.
• A deposit over 10 percent before permits are filed.
• No license number on the proposal or the website.
• A schedule that starts construction next month in a city where review takes four.
• A plan that ignores egress. Basement ADUs need a legal bedroom window, and our egress window guide explains what that costs when it is missed.

How ENF builds ADUs

We are a general contractor, not an ADU-only shop, and we build them as part of our additions and ADU work across Snohomish and King County: detached units and basement conversions in Seattle, hillside and daylight-basement units in Bothell, and units near the light rail in Lynnwood. The first visit is a feasibility read, the estimate is a written, itemized fixed number, Eduard files the permit and is on site, and every exclusion is listed before you sign.

Start with a feasibility visit →