Seattle's accessory dwelling unit rules used to live at 23.44.041. That section no longer exists. 127211 repealed it outright with effect from 30 June 2025 and moved everything into a new SMC 23.42.022. Ordinance 127376, part of the One Seattle Plan, then rewrote 23.42.022 again with effect from 21 January 2026. Anything citing the old section is describing rules that have been off the books for over a year.
The standards, in one place
| Standard | What the code says |
|---|---|
| How many | No more than two per lot. Attached, detached or stacked, in any combination. The old express allowance for two detached units in a single structure was deleted by Ordinance 127376. |
| Where | Allowed as a housing use in all zones where housing is allowed. Must be on the same lot as the principal dwelling. |
| Size | 1,000 sq ft of with two bedrooms or fewer. 1,200 sq ft with three or more bedrooms. |
| Size, narrow case | 1,500 sq ft where all three hold: the lot is in an , it is in a frequent transit service area, and it has not been purchased for more than $1,000 in the past twenty years. |
| Not counted in size | Up to 250 sq ft of attached garage, all underground stories, and up to 35 sq ft dedicated to long-term bicycle parking. |
| Height | The same as the permitted height for a principal dwelling. In , 32 ft plus pitched roof allowances, or 42 ft plus allowances if a bonus condition is met. |
| Rear | In Neighborhood Residential, 5 ft from a rear lot line, or none at all where that line abuts an alley. Front and side setbacks still apply as normal. In other zones the ADU takes the underlying zone's setbacks. |
| Separation | In Neighborhood Residential, minimum 5 ft between structures containing floor area, measured structure to structure, with eaves allowed to project 2 ft into it. The old eave-to-eave rule and the 3 ft figure for other zones were both deleted by Ordinance 127376. |
| Parking | None required. Anywhere in the city. |
| Owner occupancy | Not required, and state law prohibits reinstating it. |
| Everything else | Same development standards as a principal dwelling unit, including , which caps footprint at 50 percent of the lot in most Neighborhood Residential development. |
Size: the cap and what it excludes
The headline is 1,000 square feet, and a third bedroom lifts it to 1,200. That extra 200 square feet is one of the very few free upgrades in the code, so if the plan is close to three bedrooms it is worth going the whole way.
The exclusions count for more. in Seattle excludes all underground stories, up to 250 square feet of attached garage, and 35 square feet of long-term bicycle parking. A cottage with a full basement can therefore be a good deal larger than 1,000 square feet in real living space while still complying. Whether a basement is practical is a groundwater and excavation question rather than a zoning one, but the zoning does not stand in the way.
The 1,500 square foot allowance applies to very few lots
All three conditions have to hold at once. The lot must be in a Lowrise zone. It must be in a frequent transit service area. And it must not have been purchased for more than $1,000 in the past twenty years.
Council central staff described this as a provision for legacy homeowners in lowrise zones near transit whose lot has not sold in two decades. A house bought any time since 2006 doesn't qualify. If a builder cites this allowance, check the purchase date before anyone designs to it.
Height: the largest change
Backyard cottages in Seattle used to be capped somewhere between 14 and 18 feet depending on configuration, later 20 feet. That limit has been removed. An ADU now takes the height limit of a principal dwelling, which in Neighborhood Residential zones means 32 feet plus pitched roof allowances, and 42 feet plus allowances where a bonus condition is met. The pitched roof allowance for a principal structure lets the ridge extend up to 5 feet above the limit at a minimum 4:12 slope.
The 42 foot allowance needs one of four conditions, and two of them are out of reach for an ordinary backyard cottage because they require stacked dwelling units. The realistic routes are the tree one and the density one:
- Retain a tree, or reach a tree point score by planting or preserving medium and large trees producing at least 10 percent canopy coverage at maturity.
- Three or more principal dwelling units on the site with a front setback of at least 20 feet.
- Stacked dwelling units within a quarter mile of a school where at least a quarter of them have three or more bedrooms and 1,050 square feet or more.
- Stacked dwelling units on a lot meeting a Green Factor score of 0.6 or higher.
The tree option involves a trade-off. Keeping a large tree allows ten feet of extra height, but under the tree code it also reduces buildable ground, so it needs to be weighed on the actual lot. In an , ADU height instead follows the Lowrise limit for the applicable dwelling unit type, which is 32 feet in LR1 and 40 feet in LR2 and LR3, rising to 50 feet in LR3 inside urban centers, urban villages and station areas. The old rowhouse and townhouse category was replaced by Ordinance 127376.
The practical effect is large. Going from 18 feet to 32 feet turns a single story cottage into a genuine two story house on the same footprint: same foundation, same roof, same trench, roughly twice the floor area.
The pre-approved plan catalog predates the height change
The pre-approved DADU designs predate the height change. The repealed section capped a detached unit at between 14 and 18 feet depending on lot width, and the catalog was drawn under it, so the designs are optimized for a constraint that no longer exists. They remain valid and they still get a faster review. A refreshed catalog is in progress: the City extended the call for designs to 15 July 2026, but we could not find a published release date, so do not plan around one.
The fast track also has two eligibility limits that are easy to overlook: the property must have no on it at all, and total ground disturbance including driveways must stay under 750 square feet. The second limit is easy to exceed once a driveway is included.
Converting an existing garage
SMC 23.42.022 allows an existing accessory structure to be converted into a detached ADU notwithstanding the applicable lot coverage, yard and setback provisions, with director waivers handled as a . Converted structures must comply with the minimum housing standards at SMC 22.206.020 through 22.206.140. Read that as a floor rather than a ceiling: it is a minimum the conversion has to reach, not a substitute for the building, residential and energy codes, which still apply.
That clause can save a lot of money. If you have a garage built in 1948 sitting three feet off the side line, it is : lawful when built, non-compliant now. A new building on that footprint could not be permitted. A conversion of the existing one can be. You keep the existing position and foundation and avoid paying for new ones.
Attached ADUs inside structures that existed before 23 July 2023 may also exceed the size cap, which is the equivalent relief for a large old basement.
Three changes worth checking
1. Your cottage counts toward your floor area ratio
is the total chargeable floor area on the lot divided by the lot size. It is the number that decides how much building you are allowed in total, across every structure.
ADU floor area counts toward it. Check this carefully: an earlier draft of the Seattle code went the other way, and that draft is still online. The signed Council Bill 120993 contains no floor area exemption for accessory dwelling units, and both SDCI's Neighborhood Residential summary and the Office of Planning's January 2026 standards say the same thing in the same words: ADUs count toward allowed density, FAR and lot coverage.
The document that says otherwise is a pre-adoption compilation of chapter 23.44 which states on its own first page that it shows the code as of 30 June 2025. It carries an exemption at 23.42.022.C.3 that did not survive into the adopted ordinance. If a builder or a plan set cites that exemption, that draft is the source.
The house and the cottage share one allowance. The worked example below is the same two cases in words.
Worked example: how much floor area is left
A 5,000 square foot Neighborhood Residential lot achieving a density less dense than one unit per 4,000 square feet is allowed 0.6 floor area ratio. That is 3,000 square feet of chargeable floor area for the entire property.
An existing 1,800 square foot house leaves 1,200 square feet. A 1,000 square foot cottage fits, with 200 square feet to spare, and you have now used your whole allowance. A 2,300 square foot house leaves 700, and the cottage has to shrink by 300 square feet or the house has to give something up.
Three provisions allow more room. Underground stories are excluded from gross floor area for the size cap. Lots under 5,000 square feet are allowed up to 2,500 square feet of chargeable floor area, or the FAR figure, whichever is greater. And the ratio climbs with density. Reaching one unit per 2,200 square feet moves you to 1.0, and stacked units, courtyard developments in frequent transit areas and lots meeting the school-proximity or Green Factor conditions all sit on higher rungs again, up to 2.0.
2. Your cottage uses one of your unit slots
Seattle counts ADUs toward the lot's maximum. In Neighborhood Residential the base allowance is one unit per 1,250 square feet for attached and detached units, and one unit per 600 square feet for stacked flats.
The headline "four units" and "six units" figures come with lot size conditions that are easy to miss and that run the opposite way to expectation. Four units is available only on a lot smaller than 5,000 square feet. Six units only on a lot smaller than 7,500 square feet, and then only either within a quarter mile walking distance of a , or, beyond that distance, where at least two of the principal dwelling units are low-income units under an instrument enforceable by the City. All of those exceptions also require the lot to be clear of riparian corridors, wetlands and their buffers, submerged lands and shoreline setbacks, and designated steep slope non-disturbance areas.
Build two detached cottages and you have used two of your four. SDCI also states that only two of multiple units on a property may be ADUs. If there is any chance you might later do something larger with the property, decide deliberately, not by accident, before you spend unit slots on accessory units.
3. You can sell it as a condominium but you cannot split the land
SMC 23.42.022 requires the ADU to sit on the same lot as the principal dwelling, and SDCI states plainly that an ADU may not be separated from the principal unit through subdivision. So a is not the route.
An is. State law bars jurisdictions from prohibiting the sale of a condominium unit independently of the principal unit where it was originally built as an ADU, and Seattle's code carries the same language. It is a title and survey exercise rather than a permit: you record a declaration and a survey map under the Washington Uniform Common Interest Ownership Act. That also creates a common interest community with a budget, a reserve study and governance obligations, and it brings condominium lending rules onto both units.
If the economics of your project depend on selling the cottage separately, price that conversion before you build, not after. It is a cost that is easy to leave out of the budget.
Permitting: what to expect
A new detached unit and an attached unit inside an existing house are both handled as a construction permit for addition or alteration. Legalising an existing unauthorized unit is a construction permit to establish use. On top of that sit a separate electrical permit, a plumbing permit from Public Health, and possibly a permit. Required attachments include the King County sewage treatment form, and a rat abatement declaration if you are removing an accessory structure.
The review procedure is straightforward. A backyard cottage is a , and SDCI has stated there is no public comment period and no appeal opportunity to the Hearing Examiner or City Council. A neighbor who objects has no route to stop it. Design review does not apply at this scale, and state law independently bars design review for ADUs that is more restrictive than for principal units.
Realistic permit timelines
SDCI publishes its own performance data and is unusually candid about it. The department states that the total calendar time an applicant experiences is roughly twice the calendar days in City control.
Against a 30 day goal, the 75th percentile figure in City control was 60 days for a pre-approved DADU plan and 64 days for a single family addition or alteration. Doubled, that is a realistic four to six months from intake to issuance for a custom cottage, before construction starts and before you count the utility's own timeline, which runs in parallel and is often longer.
On fees, SDCI raised construction permit fees roughly 18 percent for 2026. The base review rate is $292 an hour, land use review is $551 an hour, and a 5 percent technology fee sits on top of everything. SDCI's own worked example shows a 1,500 square foot house permit moving from $5,759 to $6,853.
What is no longer true
A short list of things that appear in guidance still circulating, and are now wrong for Seattle.
- "You may only have one ADU." Two.
- "You have to live on site." Removed in 2019 and now prohibited by state law.
- "A backyard cottage is capped at 20 feet." It takes the principal dwelling's height.
- "You need an off-street parking space." Not for an ADU.
- "You need a 3,200 square foot lot." That minimum sat in the repealed section and there is no ADU-specific minimum lot size in the adopted ordinance. Do not confuse it with the separate rule that new lots created after the ordinance must be at least 5,000 square feet, which is about subdividing, not about building.
- "ADU floor area is exempt from FAR." See above. Treat this as the reverse of the current position and confirm with SDCI.
- "Accessory structures can only cover 40 percent of the rear yard." That rule lived in the old chapter 23.44 and was repealed with it. The adopted chapter has no coverage-of-setback percentage at all: it caps lot coverage at 50 percent and governs what may sit in a setback by structure type and dimension instead. The 40 percent figure survives online only in the same pre-adoption draft that carries the FAR exemption.
- "The entrance cannot face the street." The old ADU-specific rule, which kept entrances 10 feet off side and rear lot lines, died with the repealed section, and state law bars entry door restrictions stricter than those for principal units. But do not read that as no rule at all: SMC 23.44.140 now requires a pedestrian entry on a street-facing facade for any dwelling unit within 40 feet of a street lot line, and a pedestrian access at least 3 feet wide to the pavement for each unit. Those apply to ADUs through the same-standards provision.
Zoning is only half the question. What the code allows and what your particular back yard allows are different things, and trees, slopes and drainage stop more Seattle cottages than the land use code does. That is Part 4.
Sources
- Seattle Ordinance 127211, effective 30 June 2025, repealing SMC 23.44.041 and creating SMC 23.42.022; Ordinance 127376 (Council Bill 120993), effective 21 January 2026, rewriting SMC 23.42.022 as part of the One Seattle Plan.
- SDCI, Accessory Dwelling Units, and SDCI's Neighborhood Residential zone summary.
- Seattle Office of Planning and Community Development, Updated Neighborhood Residential Zone Standards, January 2026.
- SDCI Construction Permit Performance dashboard, and the SDCI 2026 Fee Summary and Fee Subtitle.
- RCW 36.70A.681 and RCW 36.70A.680, the state ADU requirements, and RCW 36.70A.696 for definitions.
Read from primary documents in August 2026. Seattle's zoning code changed twice in the eighteen months before this was written and fee schedules reset every January. Confirm anything you intend to rely on with SDCI for your specific address. General information from Hanco Electric, not legal or design advice.