Building an ADU in Seattle: What Homeowners Need to Know in 2026

Accessory dwelling units have gone from a niche option to one of the most common upgrades homeowners across Seattle and the Eastside ask us about. A state law change in the last few years removed most of the old roadblocks, and rental income, aging parents, and adult kids moving back home are all pushing more people to look seriously at building one. Here's what's actually allowed, what it costs, and how the process works in 2026.

What Changed: Washington's New ADU Law

Washington's HB 1337 requires cities in urban growth areas — that includes Seattle, Bellevue, Kirkland, Redmond, and Bothell — to allow at least two ADUs on any lot zoned for a single-family home. It also strips out two of the biggest obstacles that used to kill projects before they started: cities can no longer require the owner to live on the property, and they can't impose parking requirements on an ADU that are stricter than what applies to the main house.

That's a real shift from a few years ago, when an owner-occupancy covenant was often required and a missing parking space could sink a permit application. Local zoning still governs setbacks, height limits, and lot coverage, and each city is at a different stage of updating its code to match the state law, but the direction is the same everywhere in our service area: more flexibility, fewer exemptions needed.

What this means in practice for most homeowners:

• Up to two ADUs allowed per lot (a detached unit and an attached or basement unit, for example)
• No requirement that you live in the main house or the ADU
• No mandatory off-street parking space for the ADU
• Standard zone height and setback rules still apply — a lot with existing constraints may still fit only one unit

The rules vary slightly by jurisdiction in how quickly they've been formally adopted, so the first real step on any project is a quick zoning check on your specific address before you fall in love with a floor plan.

A well-planned ADU interior can live as large as a downtown apartment, just in your own backyard.

Which Type of ADU Makes Sense for You?

There are three basic paths, and they land in very different price and complexity brackets. A detached ADU (DADU) — a standalone backyard cottage — gives you the most privacy and flexibility, but it's ground-up new construction: new foundation, new utility runs, new everything. An attached ADU adds square footage onto the existing house, which saves on some site work but usually means tying into the home during construction. A basement or garage conversion reuses space you already have, which is the cheapest route on paper, but older basements often need new plumbing, electrical, insulation, and egress windows to meet code — costs that add up fast once the walls are open.

The right answer depends less on preference and more on what's actually on your lot: how much yard space you have, where your sewer line runs, whether your basement ceiling height and window wells can meet egress requirements, and what you're trying to get out of it — a rental unit, space for family, or long-term flexibility for the property.

What Does It Cost?

Costs vary a lot by type, size, and site conditions — a flat lot with easy utility access is a different project than a sloped lot that needs retaining work. These are the ranges we see across Seattle and the Eastside in 2026:

Project Type	Estimated Range
Basement or garage conversion	$100,000 – $200,000
Attached ADU addition	$100,000 – $225,000
Detached ADU (new construction), 600–900 sq ft	$250,000 – $500,000+

Foundation work, utility hookups (especially a sewer upsize), and permit fees are usually the line items that push a detached ADU toward the top of that range. On a conversion, the surprises tend to hide in egress and mechanical upgrades rather than the finish work.

Permits and Timeline

Permitting is faster than it used to be if you go with a pre-approved plan set — Seattle's program can turn around a permit for a detached ADU in as little as 2–6 weeks. A custom-designed unit goes through standard plan review instead, which typically runs 2–4 months depending on the jurisdiction and how complete the submittal is. From there, construction itself generally takes 4–6 months, so plan on 7–12 months total from first sketch to move-in for a detached unit — conversions and attached additions are usually faster.

• Site plan and survey to confirm setbacks, lot coverage, and utility locations
• Permit submittal — pre-approved plans or custom design review
• Plan review and any correction cycles with the city
• Construction
• Final inspection and occupancy

Every city in our service area handles this a little differently, and Seattle, Bellevue, Kirkland, Redmond, and Bothell are each at a different point in updating their code to the state requirements — so the actual timeline depends on which jurisdiction your property sits in.

Is an ADU Right for Your Property?

Before you get attached to a design, it's worth confirming the basics: how much of your lot is already built on, where your setbacks fall, what your sewer and water service can support, and whether any trees on the property are protected. A lot that looks plenty big on paper can still be tight once setbacks and an existing driveway are accounted for.

Beyond the legal checklist, think about what you actually want out of it. A detached unit rented long-term can offset a mortgage payment; a basement conversion for a parent or adult child solves a housing problem without building new; either way, a well-built ADU also adds real resale value to the property, not just short-term utility.

Working With the Right Contractor

An ADU touches everything from foundations to finish carpentry, so it's a different scope than a single-room remodel. We walk every property in person before quoting anything, because the honest answer on cost and feasibility depends entirely on what's actually on your lot — not a generic per-square-foot number. If you're weighing a detached cottage against a basement conversion, that's exactly the conversation to have before you spend money on architectural drawings.