Seattle ADU Rules: What You Can Build on Your Lot

By Mustapha Math, Licensed WA Real Estate Agent · The Moose Group · Published August 18, 2026 · Updated August 18, 2026

Almost every week someone in Beacon Hill, Columbia City, or Rainier Beach asks our team some version of the same question. They have a big backyard, or a daylight basement doing nothing, and they want to know what the city will actually let them build back there. It is a good question, and the honest answer has two halves. The citywide framework is genuinely friendly to accessory dwelling units. The parcel-level details are where projects get complicated, and where we see homeowners get bad information.

This guide is our attempt to give you the first half clearly and to be straight with you about the second half. We are real estate agents, not architects and not permit expediters, so where a number depends on your zone we will tell you that instead of guessing. Everything cited here comes from the city's own SDCI accessory dwelling unit page, its published tip sheets, or Washington state law, and we link to all of it so you can read the source yourself.

What Seattle Means by ADU, AADU, and DADU

An accessory dwelling unit is a separate living space either inside a house or elsewhere on the same property as a house. SDCI defines the two flavors in one sentence each. A legally permitted unit in the home is an attached accessory dwelling unit, or AADU. A legally permitted unit on the property but not within the home is a detached accessory dwelling unit, or DADU, which most people around here just call a backyard cottage.

Two words in those definitions carry a lot of weight. The first is separate. A real ADU has its own kitchen, its own bathroom, its own sleeping space, and a lockable entrance door. A bedroom with a mini fridge and a hot plate is a bedroom. The second is legally permitted. SDCI is explicit that these units may not be subdivided from the main house and are not legal unless they have been established through a permit process. That distinction matters enormously at resale, and we get into why in our companion piece on whether an ADU adds value in South Seattle.

One more definition worth knowing, because it comes up constantly. SDCI treats tiny houses on wheels like camper trailers, and you cannot live in one on a lot inside Seattle city limits. A tiny house on a permanent foundation, on the other hand, is considered a DADU and follows DADU rules.

How Many ADUs Seattle Allows on One Lot

Two. That is the ceiling, and SDCI states it about as plainly as a city agency ever states anything: while some property may be large enough to have four or more units, only two of them may be ADUs.

Read that carefully, because it is doing two jobs. It caps accessory units at two, and it acknowledges that a lot might legally hold more homes than that through other means, such as townhouse or rowhouse development in a zone that allows it. Those additional homes would be principal units under a different part of the code, not accessory units. If your interest is in a larger redevelopment rather than an accessory unit, that is a different conversation and a different set of rules.

Washington state law backs the two-unit floor. RCW 36.70A.681, enacted in 2023, requires cities and counties to allow at least two accessory dwelling units on lots in zones that permit single-family homes inside an urban growth area, in any of three configurations: one attached plus one detached, two attached, or two detached. Seattle sits inside an urban growth area, so that statute applies to the city.

What Seattle Does Not Require

This is the part that surprises homeowners most, and it is the reason ADUs pencil in Seattle when they do not in a lot of other cities. SDCI states that certain standards do not apply to accessory dwelling units, naming three specifically: parking, street improvements, and mandatory housing affordability contributions.

Each of those is worth a sentence. No required parking means the city will not force you to add a stall for the new unit, which is what kills backyard cottages on narrow lots elsewhere. No required street improvements means you are not on the hook for building sidewalk or curb across your frontage as a condition of the permit, and state law reinforces this by barring cities from requiring public street improvements as a condition of permitting an ADU. No mandatory housing affordability contribution means you skip a fee that applies to many other kinds of residential development.

There is one wrinkle in the parking rule that the city's tip sheets flag and that we have watched trip people up. Parking is not required for the new unit, but if your property currently has an off-street parking space that was required when the house was built, you generally cannot simply delete it to make room for the cottage. You would need to replace it elsewhere on the lot in conformance with the code. Worth asking about early if your only flat ground is the driveway.

What State Law Changed, and What It Did Not

Washington passed a statewide accessory dwelling unit law in 2023, codified at RCW 36.70A.681. It works as a floor, not a ceiling. It tells cities what they may not do, which means your local rules can be more generous than the statute but not more restrictive on the items it covers. Here is what it actually says on the points homeowners ask about.

That last row is the one South Seattle homeowners should read twice. Our part of the city has a great deal of mapped steep slope, landslide, and wetland area, especially along the ridges east of Rainier Avenue S and the western face of Beacon Hill. If your parcel carries a critical area designation or falls in a buffer, the statewide protections in that table do not carry over to it, and you are back to Seattle's own environmentally critical areas review. That is not a reason to give up. It is a reason to find out early.

The Numbers We Are Not Going to Guess At

We could fill this section with confident-sounding figures. We are not going to, and here is why. Seattle's ADU standards live in the Land Use Code and have been in motion since the state law landed, and the specific caps depend on your zone. A wrong number on a page about what you are allowed to build is worse than no number at all, because someone designs around it.

What SDCI says at the general level is that accessory dwelling units are restricted in size, generally count toward density limits, and must comply with the property zone's height and location standards. So there are caps. We are simply telling you to get yours from the source rather than from us.

Specifically, treat these four as parcel-level questions for SDCI or a licensed designer:

Maximum floor area. There is a cap and it varies by zone and unit type. State law tells us the cap cannot be set below 1,000 square feet, which is a useful floor to know, but the figure that governs your permit is the one Seattle applies to your zone.

Height. Your zone's height and location standards apply. State law sets a 24 foot floor for roof height limits with a proviso, but confirm what Seattle applies to your parcel.

Lot coverage and rear yard coverage. These have historically been the binding constraint on detached units in our neighborhoods more often than square footage was. They also interact with tree protection rules.

Minimum lot size. Older guidance and current state law point in somewhat different directions here, which is exactly the kind of gap you want a designer to resolve for your address rather than a blog post to resolve for a whole city.

The governing code section is Seattle Municipal Code 23.42.022, which SDCI names on its ADU page. The city also publishes Tip 116A for attached units and Tip 116B for detached units, both worth reading, with the caveat that tip sheets carry a revision date and the state law is newer than some of them. If a tip sheet and the code disagree, the code wins, and SDCI is the office that will tell you which applies.

The Permit Path, Start to Finish

Step 1: Find out what your parcel allows

Zoning, lot dimensions, easements, and any critical area mapping. Why it matters: everything downstream is shaped by this, and it is the cheapest step by a wide margin.

Step 2: Choose attached or detached

The answer is usually written into your house and your yard rather than your preference. We walk through the tradeoffs in detail in AADU vs DADU in Seattle. Why it matters: the two paths differ in cost, timeline, disruption, and what the finished unit is worth.

Step 3: Decide between a standard plan and a custom design

The city's Office of Planning and Community Development maintains a set of pre-approved DADU designs through its ADUniverse resource, created by local designers and architects. SDCI says that in most cases a standard plan can get you a permit in just 2 to 6 weeks. A custom design gives you control over layout, light, and how the cottage sits on a sloped lot. Why it matters: permitting time is carrying cost, and on a straightforward lot the standard-plan route is hard to beat.

Step 4: Pull the right permit

SDCI lists three paths. Adding a unit within an existing house requires a construction addition or alteration permit. Building a detached unit also requires a construction addition or alteration permit. Legalizing an existing unpermitted unit requires a construction permit to establish use. You may also need an electrical service change or new service from Seattle City Light, plus separate electrical, plumbing, or side sewer permits depending on scope. Why it matters: the legalization path is a real, documented option, and a lot of South Seattle houses have a basement unit that predates anyone's memory.

Step 5: Budget for the charges that are not construction

SDCI reports all ADU permits to King County for sewer treatment capacity charges, and you will fill out a form acknowledging those charges during your application. The county bills that charge over a long period rather than all at once. Permit intake and plan review fees are separate, and your assessed value will rise once the new square footage is recorded. Why it matters: these are predictable and they are routinely left out of first-draft budgets. Our companion article on the cost to build a DADU in South Seattle goes deeper on the money side.

Step 6: Build, inspect, and then decide how you will use it

Inspections happen throughout, and a tenant cannot occupy the unit until the final inspection is approved. After that the unit is yours to use, and owners split roughly three ways: rent it, house family in it, or move into it and rent the main house. If renting is your plan, our guide to renting out your Seattle ADU covers what comes next. Why it matters: the intended use should shape design decisions well before framing starts.

How This Plays Out in South Seattle

Citywide rules meet very different ground depending on which of our neighborhoods you are in, and after helping more than 150 families across the South Seattle real estate corridor we see clear patterns.

Beacon Hill is full of early 1900s homes on deep lots, and the daylight basements along the slope are natural attached-unit candidates. We wrote a full piece on Beacon Hill ADU and DADU investment if that is your neighborhood. Columbia City has the walkable core and the transit that renters want, with lot geometry that varies block to block. Mount Baker brings grade changes and larger lots, a combination that can favor a detached unit but also raises engineering cost. Rainier Beach tends to offer the most yard per dollar in the city, which is exactly what a backyard cottage needs.

For a neighborhood-by-neighborhood read on where accessory units make the most sense, see our South Seattle ADU neighborhood guide.

One note for anyone whose search has drifted south of the city line. Kent, in King County, sets its own accessory dwelling unit rules through its own municipal code, and nothing on this page applies there. If you are weighing the two markets, our Kent versus South Seattle comparison and our Kent housing market overview are the right starting points, and you would want to confirm ADU rules with the City of Kent directly.

Who to Call for What

We say this to every client and it saves a lot of wasted time. Permitting, zoning, code interpretation, and anything involving a dimension go to SDCI and to a licensed architect or designer. Financing the build goes to a lender, and it is genuinely a lender conversation rather than an agent one. Tax questions, including what the added assessed value does to your bill and how rental income is treated, go to a CPA. Landlord and tenant questions go to an attorney who practices in that area. What the unit does to your home's value, who will want to buy it, and how to position the property go to us.