How Big Can a DADU Be in Seattle?

In Seattle, a DADU can be up to 1,000 square feet with two bedrooms or fewer, and up to 1,200 square feet with three or more bedrooms, in every zone where accessory dwelling units are allowed. You can have two ADUs per lot in addition to your primary home, and a detached unit can reach 32 feet in most Neighborhood Residential zones.

Those are the headline numbers. The more useful answer is that square footage is rarely what limits a backyard cottage — setbacks, lot coverage and trees usually bind first. Here's how the envelope actually gets decided.

What the current ordinance allows

Seattle's middle housing changes arrived in two waves: an interim ordinance effective June 30, 2025, then the permanent Neighborhood Residential update, which took effect January 21, 2026. That second ordinance repealed and replaced the whole Neighborhood Residential chapter (SMC 23.44) and amended the ADU standards in SMC 23.42.022, so guidance written before 2026 — including some of the City's own older tip sheets — describes rules that no longer apply. The standards that matter to a homeowner planning a backyard build:

• A 1,000 sq ft maximum for an ADU with two bedrooms or fewer, and 1,200 sq ft with three or more, in all zones where ADUs are allowed — with up to 1,500 sq ft available in Lowrise zones near frequent transit on long-held lots.
• Two ADUs per lot permitted in addition to the primary unit, on all lots.
• Height up to 32 feet in most Neighborhood Residential zones, including for a rear-yard unit, and higher in Lowrise zones.
• A 5-foot rear setback for accessory dwelling units specifically — and none at all where the rear of the lot abuts an alley.
• ADU setback and height standards no more restrictive than those applied to the primary home.
• Increased lot coverage allowances.
• Density set by lot area instead of a flat unit cap — one dwelling unit per 1,250 sq ft — with exceptions allowing four units on any lot under 5,000 sq ft, and six on a lot under 7,500 sq ft within a quarter-mile walk of a major transit stop. Only two of the units on a lot may be ADUs, and ADUs count toward the total.
• Detached, attached and stacked flat configurations all permitted.

That combination is a genuine shift. A 1,000 sq ft detached unit at 32 feet is a two-storey cottage with room for two real bedrooms, not a studio over a garage. It also means the constraint most homeowners assume they're fighting — “can I fit enough square feet?” — is usually not the one that decides their project.

The companion question is how many buildings, not how big. Two ADUs or a fourplex works through the density arithmetic on a Seattle lot, including why ADU floor area now counts toward your floor area ratio.

What actually limits the building

The 1,000 sq ft cap is a ceiling. Your buildable envelope is the floor and the walls, and it comes from the standards applied to the lot as a whole.

Setbacks

In Neighborhood Residential zones, SDCI's summary of the standards runs roughly: 15 feet in front for one to two units and 10 feet for three or more; side setbacks of 5 feet average with a 3-foot minimum, reduced to 3 feet in frequent-transit areas; and rear setbacks of 15 feet for one to two principal units, 10 feet for three or more, dropping to zero where the lot abuts an alley.

Accessory dwelling units get their own, far friendlier rear number: 5 feet, and none at all if the rear of the lot abuts an alley. That is the most useful line in the current code for a backyard cottage, and it is the one most pre-2026 articles get wrong. Five feet against a neighbor versus zero against an alley is still a meaningful difference on a standard 5,000 sq ft lot, so the alley remains the first thing we look at when someone sends an address.

Lot coverage

Neighborhood Residential lot coverage generally runs to 50% (60% for stacked units), and an ADU counts toward it, along with density and floor area limits. Your existing house, garage, and any covered areas are already consuming that budget. On a lot with a large footprint house, coverage — not the 1,000 sq ft cap — is what sets the size of the cottage you can build.

Trees, slopes and drainage

Seattle's tree protection rules and any critical-area designations sit on top of all of this, and a single protected tree in the wrong part of a backyard can relocate or shrink a unit — a 24-inch tree carries a 24-foot protected radius, and that circle comes out of your buildable area. Seattle's tree protection rules and your DADU works through the tiers, the removal tests and the setback and height flexibility you get for keeping a big tree. Slope and drainage do the same thing to the site work budget, if not to the envelope — see drainage, excavation and site work and building on a difficult lot.

Height: what 32 feet gets you

Thirty-two feet comfortably accommodates two storeys with generous ceilings. The current Neighborhood Residential rules also raise the limit to 42 feet on a lot that retains a Tier 1 or Tier 2 tree — a deliberate trade of height for canopy, and one that can recover square footage a protected tree cost you on the ground. The practical design question is usually not whether you can reach the limit but whether you want to: a taller unit changes what your neighbors see, what the stair consumes on the ground floor, and what the roof costs.

Because ADU height standards are now no more restrictive than those for the primary home, the older pattern of a deliberately squat backyard unit is no longer required. How that translates into a plan is the subject of choosing an ADU floor plan, and you can see real examples on our floor plans page.

What Seattle no longer requires

Three requirements that used to shape backyard builds are gone, and homeowners are often still planning around them:

• No off-street parking requirement for an ADU.
• No street improvement requirement.
• No owner-occupancy requirement, following state reform.

ADUs are also exempt from Mandatory Housing Affordability contributions. Parking in particular still drives a lot of outdated advice — see ADU parking rules in Seattle for the detail, and owner occupancy in Washington for what the state changed.

A note on where these rules apply

Everything above is Seattle city limits. Washington's statewide reform sets a floor that other jurisdictions have to meet, but Bellevue, Redmond, Kirkland, Bothell, Everett, Monroe and unincorporated county each implement it differently — different caps, different setbacks, different design standards. If your lot is outside Seattle, read Washington's ADU laws and the county-by-county breakdown instead of this page.

One consequence of the 2026 rewrite worth flagging: Seattle's pre-approved DADU designs were selected in 2020, against a tighter envelope than the code now allows. They still comply, but they do not use the larger footprint, the extra bedroom count or the five-foot rear setback available today — which on a small lot is exactly where the usable space is. More in are Seattle's pre-approved DADU plans worth it.

Find your actual envelope

Codes change, and the version that governs your build is the one in effect when you apply — confirm current standards with SDCI or get an opinion letter before you commit to a design.

The faster route is to send us the address through the contact form. We'll tell you what your zone allows, whether the alley or a tree is going to be the deciding factor, and what the unit that actually fits would cost to build — see what a DADU costs in Seattle for the ranges and the DADU permit process for what approval involves. Once you know the size, what it does to your property taxes is the next number worth understanding.