Can I build a DADU on this lot in Seattle?

6 min read · 8 sections

Most single-family lots in Seattle can take a DADU — but “most” is not “yours,” and the handful of things that disqualify a lot are checkable in about fifteen minutes. Do that check before you write the offer, not after.

Why 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.

The fifteen-minute check#

In order. Any one of these can end it, so run them in the order that kills fastest.

1. 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.
2. Check lot size against the minimumSmall lots are the most common quiet disqualifier. Get the square footage from the King County Parcel Viewer rather than the listing, which is often rounded or wrong.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.

VERIFY BEFORE PUBLISHING — These steps name the checks, not the numbers — deliberately, since caps, setbacks and tree rules change and state middle-housing law is still rolling out city by city. Re-read this quarterly and keep it procedural unless a figure can be sourced and dated.

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#