Guides · Seattle
Can I build a DADU in Seattle? The 2026 rules and a 15-minute lot check
Yes, on most Seattle house lots. Since June 30, 2025, the city allows up to two ADUs on a lot, each up to 1,000 square feet, or 1,200 with three or more bedrooms. You do not need a parking space, you do not have to live there, and there is no minimum lot size. What stops a DADU is the lot itself: coverage, setbacks, slopes, trees and the sewer line. Check those before you write the offer.
What a DADU is#
The rules in Seattle, October 2026#
Seattle rewrote its ADU rules in 2025 and its house zone in January 2026. Most house lots are now in one Neighborhood Residential (NR) zone. Some older pages, including the city's own 2023 tip sheet, still show rules that no longer apply.
| Rule | What the code says now |
|---|---|
| How manySeattle Municipal Code 23.42.022; RCW 36.70A.681 | Up to two ADUs on a lot, attached or detached |
| SizeUnderground space and up to 250 sq ft of attached garage do not count | Up to 1,000 sq ft with up to two bedrooms, or 1,200 sq ft with three or more |
| Minimum lot sizeOrdinance 127211 repealed the old 3,200 sq ft minimum. Lot coverage and floor area still apply | None since June 30, 2025 |
| Lot coverageSMC 23.44.080 | 50% for everything on the lot together |
| HeightA cottage inside the house's required rear yard may be held lower. Ask SDCI for your lot | Generally the same limit as a house: 32 feet in the NR zone |
| Rear setbackSMC 23.44.090 | 5 feet, or none on an alley |
| ParkingSMC 23.42.022 | None required |
| Living on siteState law bars an owner-occupancy rule (RCW 36.70A.681) | Not required |
| SellingSMC 23.42.022 and 23.24.045 | Can be sold as a condo unit, not split onto its own unit lot |
| Permit timeSDCI ADU page; city ADU report, October 2025 | 2 to 6 weeks with a city pre-approved plan; median about 160 days otherwise |
| Sewer chargeKing County capacity charge for a new ADU connection | $46.01 a month for 15 years, King County 2026 rate |
Permit fees are based on the value of the project, so there is no single fee for a DADU. The city says to expect at least a year from start to move-in.
Sources
- Seattle Municipal Code 23.42.022 (accessory dwelling units), the size, number, parking and condo rules
- RCW 36.70A.681, the state ADU law
- City of Seattle: NR zone summary, January 2026, the zone that took effect January 21, 2026
- SDCI: accessory dwelling units, permits and pre-approved plans
- City of Seattle ADU report, October 2025, permit times and how many get built
- King County: sewer capacity charge, the monthly charge for a new connection
Not sure how this applies to the house you're looking at?
Talk it through before you writeWhy this is worth checking yourself#
The city will tell you the rules. A builder will tell you the cost. Neither one is looking at the specific parcel you are about to bid on, and the disqualifiers are parcel-level: slope, trees, an existing unit, where the side sewer runs.
None of that is on the listing. It is the difference between a house that pencils as a house-hack and one that does not, and it is knowable before you write.
Sources
- Seattle SDCI, accessory dwelling units, the city's rules and permit steps
The fifteen-minute check#
In order. Any one of these can end it, so run them in the order that kills fastest.
- Find the zoneStart at the city's ADUniverse property search. It is parcel-level and free. Note the zone designation. That single field drives size, height and setback limits. ADUniverse itself warns that some sections may be out of date after the 2025 and 2026 rule changes, so read the numbers from the city code. Since January 21, 2026, most house lots are in one Neighborhood Residential (NR) zone; the old NR1, NR2 and NR3 labels are gone.
- Check what the lot size allowsThere is no minimum lot size for a DADU. Seattle repealed the 3,200 square foot rule on June 30, 2025, though some older pages still show it. Lot size still caps how much you can build. Everything on the lot counts toward 50% lot coverage and a floor-area limit. Get the square footage from the King County Parcel Viewer rather than the listing, which is often rounded or wrong.
- Work out what is already builtLot coverage is a percentage of the parcel, and the existing house plus any garage, deck or shed already eats into it. A large main house on a modest lot can leave nothing.
- Look at the rear yard and setbacksA DADU has to sit off the property lines. Measure the buildable rectangle that is left after setbacks. This is where a long narrow lot quietly fails.
- Screen for critical areasSteep slope, landslide-prone soil, wetland or liquefaction zones all trigger extra review and can end the project outright. Seattle's ECA map is the check. West Seattle, Magnolia and anything on a ravine edge deserve extra attention here.
- Check the treesThe one almost nobody checks. Seattle's tree protection rules can force a redesign or rule out a siting entirely, and a mature tree in the back corner is exactly where a DADU wants to go. Look before you fall in love with the yard.
- Find the side sewerWhere the sewer line runs, and whether the DADU can reach it, is a five-figure swing that never appears in a cost-per-square-foot estimate. Side sewer cards are public.
- Count what already existsA basement apartment may already be a permitted attached unit, which changes what else the lot can hold. Confirm whether it is permitted or merely existing. Those are very different things at resale.
Rules, programs and costs change, and they differ between Seattle and each surrounding city. Figures on this page are dated where they appear. Confirm anything specific with the relevant city or agency before you rely on it.
Is a DADU actually worth it?#
Yes. But only if you are willing to trade for it. You give up yard, you give up some privacy, and you give up a year or more of your attention to permitting and construction. What you get back is a second income attached to the place you already live. If that trade sounds bad to you, stop here; no spreadsheet will fix it.
The reason it works in Seattle specifically is the gap between renting and owning. Purchase prices and rates have pushed the monthly cost of ownership up sharply, while rents have stayed high and steady rather than spiking with them. Owning got expensive faster than renting did. That gap is the whole opportunity. It is what lets a rented unit carry a meaningful share of a mortgage that would otherwise be out of reach.
My read is that the gap is not closing soon: demand for rentals here has held up through several cycles that were supposed to end it. That is my read, not a guarantee. Anyone who tells you they know where rents go next is selling something.
What it looks like when it works#
I grew up in a house that did this. My parents converted the garage and the back yard into a second living space, and that space now operates as a licensed adult family home. It is north of Seattle, not in the city, which is part of the point.
Two things worth taking from that. First, an adult family home is licensed care, not a rental, and it carries its own state licensing and inspection requirements. It is a different undertaking than renting a cottage to a tenant. But it demonstrates the structural point: converted space is not a spare room, it is a use, and the range of viable uses is wider than most owners assume.
Second, this is not a Seattle-only idea. Land and development costs across the Puget Sound region are high enough that the math works in the suburbs too, and it is improving as transit expands. The city of Seattle is not where the demand stops.
Sources
- Seattle eases accessory dwelling restrictions ahead of state deadline, The Urbanist, July 2025
The better reason: future-proofing#
Here is how I actually think about it. A DADU is not primarily an investment. It is optionality.
The usual answer to needing more space is to sell and buy something bigger: new mortgage, new rate, new block, and every cost that comes with moving. A backyard unit is the other answer. You add the space and you stay put. You keep the house, the block, the commute, and whatever you have already paid down.
It runs the other direction too. If the kids are gone and you have more house than you need, you do not have to downsize out of the neighborhood. Convert the garage, the basement, or the lower floor, and let the part you are not using pay for the part you are.
That framing matters because it changes what a good outcome looks like. Judged purely as an investment, a DADU has to beat every other place you could put the money. Judged as future-proofing, it only has to beat moving.
Buying a home that already has one#
A backyard cottage that is already standing raises a different question: is it permitted. A permitted unit shows up in the city's records and appraises as living space. An unpermitted one is a liability you are buying, and lenders and insurers treat it that way.
Ask for the permit number before the inspection period ends. If nobody can produce one, price the risk into the offer rather than assuming it can be legalized later.
Outside Seattle#
Every city writes its own rules, and they differ more than people expect: size caps, height, how many units a lot can hold, and how long permitting takes are all local. A lot that works in Shoreline may not work the same way three blocks south in Seattle.
The eight-step check above still applies anywhere. What changes is which page you read it against: use the city's own planning department, not Seattle's, and not a builder's summary of it.
Common questions#
Can every Seattle lot have a DADU?
No. Most house lots can, but lot coverage, floor area, setbacks, critical areas and protected trees each rule out some of them. Lot size matters only through those limits. The old 3,200 square foot minimum was repealed in 2025.
Can I sell the DADU separately from the house?
Not on its own. The DADU and the main house sit on one parcel, so selling them separately means changing that first, and the route most people have heard of is closed: Seattle's code does not allow a unit lot subdivision to separate an accessory dwelling unit from its house. That leaves two. Subdividing the lot properly, which works only if each new lot can meet the minimum lot size in your zone, 5,000 square feet in Neighborhood Residential since January 2026. Or a condominium, where each unit becomes its own sellable interest. Both are real processes with cost and feasibility questions attached, and neither is a given on any particular lot. If selling the unit separately is the plan, work that out before you build, not after.
Does a DADU need its own parking?
No. Seattle's code requires no off-street parking for an accessory dwelling unit (SMC 23.42.022). Older guides and builder pages often still say otherwise. Outside Seattle, check the rules of the city your property is in.
Should I check this before or after making an offer?
Before. The checks are free and public, and finding a disqualifier during the inspection period costs you the earnest money conversation instead of nothing.
How big can a DADU be in Seattle?
Up to 1,000 square feet with up to two bedrooms, or 1,200 square feet with three or more. Underground space and up to 250 square feet of attached garage do not count.
Is there a minimum lot size for a DADU in Seattle?
No, not since June 30, 2025. Ordinance 127211 repealed the old 3,200 square foot minimum. Lot coverage and the floor-area limit still decide how much fits.
How long does a DADU permit take in Seattle?
With a city pre-approved plan, SDCI says 2 to 6 weeks in most cases. Otherwise the city's median for small projects like DADUs is about 160 days. The city says to expect at least a year from start to move-in.
Related guides
- What buying in Seattle actually costs: the Seattle buyer's guide
- Can I subdivide my lot in Seattle? What the 2026 rules allow
- Buying a condo in Seattle: read the building before you buy the unit
