What does a DADU cost to build in the Seattle area?

Detached ADUs typically run $250,000–$400,000+ in 2026, or roughly $350–$500 per square foot. A DADU is a complete small house — foundation, framing, roof, envelope, kitchen, bath and its own utility runs — so it costs more per square foot than an attached conversion.

How long does it take to build a DADU?

Plan on twelve to eighteen months end to end: two to four months of design, three to six months of permitting (longest in Seattle at SDCI), and six to nine months of construction. Permitting, not building, is usually the longest single phase.

How big can a backyard cottage be?

Size caps, height limits, lot coverage and rear/side setbacks are set by your city and vary by zone and lot. Seattle allows generous DADUs relative to most jurisdictions. Because the limits interact, the only reliable answer comes from checking your specific parcel against current code.

Do I need to add parking for a DADU?

In most Washington cities, no — recent state legislation removed most off-street parking mandates for ADUs, which is a large part of why DADU permits surged. A few jurisdictions retain requirements in specific circumstances, so we confirm your city's rules during the feasibility check.

Field Notes · September 3, 2026 · 8 min read

Building a DADU: The Backyard Cottage, Start to Finish

How a detached ADU actually gets built in the Seattle area: siting and setbacks, utility runs, the permit path, realistic costs and the twelve-month timeline from idea to keys.

Detached dwelling built by ENF General Contracting in the Seattle area

Detached ADUs went from rare to routine in the Puget Sound in about three years. Washington now leads the country in ADU permit growth, and in Seattle, ADU construction has begun outpacing detached single-family homebuilding. Here is what actually happens between “we should build a backyard cottage” and keys.

Phase one: does the yard work?

Before anything else, four things get checked against your parcel: rear and side setbacks, height limit, lot coverage, and tree retention. A DADU has to fit inside what is left after all four apply — and on a typical lot, that buildable rectangle is smaller than people expect.

Then access and utilities. Where do water, sewer and power come from, how far do they have to travel, and can the crew physically get equipment into the back yard? A cottage forty feet behind the house with no side-yard access is a different budget than one near the alley.

Phase two: design and permits

A DADU needs a full permit set — stamped drawings, structural engineering, energy code compliance, stormwater. Review times vary sharply by jurisdiction: Seattle’s SDCI is the slowest in the region and can run several months, while Snohomish County cities generally move faster. This is the phase where projects lose months, so we file early and answer correction rounds fast. Our permits guide walks the whole process.

Phase three: the build

A DADU is a complete house at small scale. Excavation and foundation, framing, roofing and envelope, utility trenching from the main house or the street, mechanical and electrical, insulation, then a full kitchen and bathroom in a footprint where every inch is planned. Six to nine months of construction is typical.

What it costs, honestly

$250,000–$400,000+ is the realistic 2026 range, and site conditions move it more than finishes do. Slope, distance to utilities, tree protection and access dominate. If that number is higher than you hoped, the garage conversion path delivers an ADU for a fraction of it — and the ADU cost guide compares every route side by side.

Why people build them anyway

Because a DADU is the rare project that is simultaneously housing for family, income in one of the tightest rental markets in the country, and appraisable square footage on land you already own. The ROI guide covers why it is the one project around Seattle that behaves like an investment rather than a lifestyle purchase.

Start with the yard

We build ADUs and DADUs across Seattle, Everett and the Eastside. The free estimate includes walking your back yard and telling you what will actually fit — before you pay anyone for drawings.

See if your yard fits a DADU →

Frequently asked questions

What does a DADU cost to build in the Seattle area?: Detached ADUs typically run $250,000–$400,000+ in 2026, or roughly $350–$500 per square foot. A DADU is a complete small house — foundation, framing, roof, envelope, kitchen, bath and its own utility runs — so it costs more per square foot than an attached conversion.
How long does it take to build a DADU?: Plan on twelve to eighteen months end to end: two to four months of design, three to six months of permitting (longest in Seattle at SDCI), and six to nine months of construction. Permitting, not building, is usually the longest single phase.
How big can a backyard cottage be?: Size caps, height limits, lot coverage and rear/side setbacks are set by your city and vary by zone and lot. Seattle allows generous DADUs relative to most jurisdictions. Because the limits interact, the only reliable answer comes from checking your specific parcel against current code.
Do I need to add parking for a DADU?: In most Washington cities, no — recent state legislation removed most off-street parking mandates for ADUs, which is a large part of why DADU permits surged. A few jurisdictions retain requirements in specific circumstances, so we confirm your city's rules during the feasibility check.

Written by

Eduard — Owner & General Contractor

Eduard owns ENF General Contracting and has built and remodeled homes across Everett, Seattle & the Eastside for over a decade. Licensed, Bonded & Insured. He runs every ENF project personally — and writes these guides from the job site. More about Eduard · (425) 217-9921