ADUs and DADUs in Seattle: The Rules Changed, and So Did the Math

If you looked into building a backyard cottage in Seattle a couple of years ago and filed it away, the file is out of date. Ordinances 127375 and 127376, adopted in December 2025, rewrote Neighborhood Residential zoning citywide, and they changed the accessory dwelling unit rules in one way that reverses the whole calculation. An ADU used to be bonus density: your house, plus up to two extra units the zoning would not otherwise have allowed. It is not bonus any more. Accessory units now count toward the same unit limit as everything else on the lot, and that limit went from one to four or more.

That sounds like a downgrade and it is the opposite. Here is what the rules actually say, what your lot probably allows, and how to think about whether a unit pays for itself.

What the ADU Rules Say Now

The accessory unit rules live in Seattle Municipal Code 23.42.022, and the current version comes from Ordinance 127376. The provisions that matter to a homeowner:

• 01No lot may have more than two accessory dwelling units. They can be attached, detached, or stacked, in any combination.
• 02A unit with two bedrooms or fewer is capped at 1,000 square feet of gross floor area. With three or more bedrooms the cap is 1,200 square feet.
• 03Three things are excluded from that floor area: up to 250 square feet of an attached garage, any story or part of a story that is fully underground, and up to 35 square feet for long-term bike parking.
• 04No off-street parking is required, for any accessory unit, anywhere.
• 05There is no owner-occupancy requirement. You can rent the house and both units.
• 06The city cannot prohibit selling a unit built as an ADU as a condominium.

Two special cases are worth knowing because they break the size caps. Converting an accessory structure that existed before July 23, 2023, such as an old detached garage, lets the director waive the ADU standards and the underlying zone's lot coverage and setback rules as a Type I decision. And an attached unit inside a principal structure that existed as of that same date can exceed 1,000 square feet outright. If you have an old garage or a big basement, the rules are friendlier to you than to someone building new.

There is also a narrow 1,500 square foot allowance, but it requires a Lowrise zone, a frequent transit service area, and a lot that has not sold for more than $1,000 in the past twenty years. Almost nobody qualifies. Mentioning it mostly to explain why you may see the number quoted online.

The Change Almost Nobody Has Caught Up To

Here is the provision that matters most, and it is one line. Code section 23.42.022.J: when calculating density, the number of dwelling units includes both accessory dwelling units and principal dwelling units.

Before, a Neighborhood Residential lot allowed one house, and ADUs were an exception layered on top, which is why the old advice was that a backyard cottage was the only way to add a unit in a single-family zone. Now the lot has a real unit count, and your cottage is one of those units. So the question changed shape. It is no longer "can I add a cottage to my single-family lot." It is "my lot allows four or more units, and what is the best thing to put in them."

That is a genuinely better position to be in. It is also worth checking the date on anything you read about Seattle ADUs, including this, because the rules moved recently and some guidance still reflects the previous code.

What Your Lot Probably Allows

Density in Neighborhood Residential zones is set in code section 23.44.060, and it is expressed as lot area per unit rather than a flat count. The general rule and the exceptions both matter.

Then there are floors for small lots, which is where the widely quoted four-and-six figures come from. A lot under 5,000 square feet can be developed with up to four units. A lot under 7,500 square feet within a quarter-mile walking distance of a major transit stop can have up to six. The same six applies beyond that walking distance if at least two of the units are income-restricted under an agreement with the city. All three exceptions require the lot to be free of critical areas: no riparian corridors, wetlands and their buffers, submerged or shoreline-setback land, or steep-slope non-disturbance areas.

Run your own lot through the common case and the picture gets concrete. A standard 5,000 square foot Seattle lot supports four units. A 6,400 square foot lot supports five. The deeper lots common in Southeast Seattle, where 7,000 to 10,000 square feet is not unusual, support five to eight. That is the part of this city where the new rules bite hardest, and it is the part I work in.

One more number to note if you are thinking about subdividing rather than building: the minimum size for a lot created after the ordinance is 5,000 square feet.

What It Costs, Honestly

This is where I have to be careful, because almost everything written about ADU costs in Seattle is published by someone who would like to build yours.

The ranges those sources quote for a detached unit run from roughly $300 to $700 per square foot, which is so wide it is not a budget, and the all-in totals they give for a 600 to 800 square foot cottage span from the mid-$200,000s to $600,000. I am not going to pick a number out of that spread and present it as the answer. What I can tell you is what actually moves it:

• 01Site work. Slope, soil, and access drive more variance than finishes do. Seattle's older neighborhoods have both.
• 02Utilities. A new detached unit usually needs its own water, sewer, and power connections, and the sewer run is the line item that surprises people. On the peat and clay soils in parts of the city, it surprises them more.
• 03Whether you are converting or building new. An existing garage conversion starts from a foundation and a shell, and the code's waiver provisions for pre-2023 structures are written to make that easier.
• 04Design and permitting. Seattle's pre-approved plan program, ADUniverse, exists to cut both. A design that has already been reviewed for code compliance moves through permitting faster than a custom one, and it removes most of the architectural fee.

The honest process is to get two or three real bids on your actual lot before you believe any per-square-foot figure, including the ones above. A number from a builder who has walked your site is worth more than every cost guide on the internet combined.

Whether It Pays

Three things belong in the arithmetic, and the third is the one people forget.

First, the rent. A detached unit in a close-in Seattle neighborhood rents for real money, and unlike a house, it comes with no commute cost to the landlord. Against a build cost in the high six figures, though, the yield is often thinner than owners expect once vacancy, maintenance, and management are honest inputs.

Second, the value. An ADU adds to what the property is worth, but rarely dollar for dollar with what it cost, and appraisers in Seattle still have a thin comparable set for houses with cottages. My guide to what your Seattle home is worth covers why that matters: if the appraisal cannot find comparables, the added value is harder to borrow against or sell into than the receipts suggest.

Third, the tax. A new unit gets assessed, and your property tax rises with the added value. On a Seattle levy rate just under 1%, a $300,000 improvement adds roughly $3,000 a year, every year. My guide to King County property taxes explains how the assessment feeds the bill. Nobody's ADU pro forma I have ever been shown included this line, and it belongs in yours.

Against all that, weigh the alternative you actually have, which is selling. My guide to the cost of selling a house in Washington has those numbers, and the August market update has the current conditions: the most inventory in any August in a decade, and a median Seattle house at $920,000. For an owner sitting on a large lot in a neighborhood that now allows four to eight units, the honest comparison is three-way. Build and hold. Build and sell. Or sell the lot to someone who will build, which in this zoning is a different and often larger number than selling a house.

Who Should Actually Do This

The cases where I have seen a unit make clear sense have a shape.

You already have the structure. An old garage, a daylight basement, or a large attached footprint that predates July 2023, which means the conversion path and its waivers are open to you. This is the least expensive version of the project by a wide margin.

You need the housing, not the yield. A parent moving in, an adult child staying local, a home office that has outgrown a spare room. The financial case does not have to close when the unit is solving a problem money would otherwise solve less well.

You are holding the property for a long time. Ten years of rent and appreciation reads very differently from three, and the build cost is front-loaded.

Where I would slow you down: if the plan is to build a cottage, hold it two or three years, and sell into a gain, the combination of build cost, added property tax, thin appraisal comparables, and the selling costs at the end makes that a hard case to win right now. And if what you actually have is a big lot in a zone that now allows six units, a single accessory unit may be the smallest version of the opportunity in front of you.

The Bottom Line

Seattle's accessory unit rules are more permissive than they have ever been: two units per lot, up to 1,000 or 1,200 square feet each, no parking, no owner-occupancy, and the right to sell one as a condominium. The rule that changed the math is that those units now count toward the lot's density rather than sitting outside it, and that density went from one unit to four, five, six, or more depending on your lot size, your housing type, and how far you are from a transit stop.

So the useful first step is not a builder quote. It is knowing what the property is worth as it stands, because every version of this decision, build and hold, build and sell, or sell the land, gets measured against that number. I will run a free, no-obligation valuation using current neighborhood data rather than an online estimate, and we can talk through which direction the numbers favor. My free Seller Guide covers the sell-side path in detail. I am a broker rather than an architect or a land use attorney, so confirm design and permitting specifics with the city or a professional before you spend money on plans.