Last verified: October 22, 2026 · Written by Marc
The short answer: Seattle’s own permit records put the median estimated project cost of a detached ADU (DADU) at $196,326 for permits issued in 2025, up from $143,017 in 2021. For attached ADUs, the 2023–2025 median is $123,069, with a much wider spread. Those are the City’s numbers, not a contractor’s. But they are not your budget either. They leave out Washington’s 10.55% sales tax on construction in Seattle, King County’s sewer capacity charge (about $8,282 over 15 years for an ADU), possible Seattle Public Utilities connection charges, design and site work. Below is how we got the figures, what they include, and how to turn them into a number you can take to a lender.
Why this guide exists. When we wrote our Seattle ADU fees guide, we refused to print builder price ranges because no neutral source backed them. Since then we found one: the City of Seattle publishes every building permit, with an estimated project cost, as open data. This page is built on that dataset. For California numbers, see our guide to ADU cost in California.
What Seattle’s permit records show about ADU cost
We downloaded the City’s Building Permits dataset on October 22, 2026. We kept the permits that the Seattle Department of Construction and Inspections (SDCI) classifies as “Accessory Dwelling Detached” or “Accessory Dwelling Attached”, issued from 2021 to 2025.
Those labels exclude permits that also build a new main house, whose cost would swamp the ADU. We also dropped records with no cost or a cost under $1,000. That leaves 1,461 ADU permits issued in 2023–2025: 1,112 detached and 349 attached.
The field we used is the City’s “estimated project cost”. The dataset describes it as based on fair market value (parts plus labor) and warns that it is the best available information to date.
Detached ADUs (DADUs): median cost by year
| Year issued | Permits with a cost | 25th percentile | Median | 75th percentile |
|---|---|---|---|---|
| 2021 | 252 | $90,479 | $143,017 | $168,547 |
| 2022 | 381 | $117,126 | $153,730 | $176,822 |
| 2023 | 432 | $144,930 | $167,664 | $188,753 |
| 2024 | 436 | $160,525 | $189,258 | $206,388 |
| 2025 | 244 | $173,226 | $196,326 | $210,315 |
Three things stand out:
- The median rose about 37% from 2021 to 2025 ($143,017 to $196,326). Part of that tracks construction prices, and part tracks the City’s own valuation table, which it updates regularly (more on that below).
- The spread is narrow. In 2025, half of all DADU permits fell between about $173,000 and $210,000. Real-world bids are rarely that tidy. That is a clue about where the number comes from.
- New structures and conversions differ less than you might think. Across 2023–2025 (median for all detached permits: $182,874), DADUs filed as “New” had a median of $185,950 (545 permits). Those filed as “Addition/Alteration”, which typically means building on or converting an existing structure such as a garage, had a median of $175,000 (561 permits). Six more were change-of-use permits with no construction.
Attached ADUs and conversions
Attached ADUs (AADUs), often basement or upper-floor conversions, show a far wider range. For 2023–2025 the median is $123,069, the 25th percentile is $50,000 and the 75th percentile is $215,812.
That spread makes sense. A basement that already has ceiling height, a window well and a bathroom rough-in is a different job from carving a unit out of finished living space.
Key fact: If your lot can take an attached unit in existing space, Seattle’s own data says the typical project costs much less than a backyard cottage. However, “typical” hides a range of more than $165,000 between the 25th and 75th percentiles, so get bids early.
Why the permit number is not your budget
Here is the catch. Under Seattle Municipal Code 22.900D.010, the SDCI Director determines the value of construction for fee purposes, using building valuation data from the International Code Council (ICC), as adjusted by the City.
SDCI’s current table, Director’s Rule 05-2025 (effective July 1, 2025), values one- and two-family construction (occupancy R-3) at $186.29 per square foot for type V-B construction, the category for most wood-frame houses, and $197.75 for type V-A.
Do the math for a 1,000 sq ft cottage: 1,000 × $186.29 = $186,290. That lands almost exactly on the 2023–2025 median for new DADUs ($185,950).
Our reading (an inference, not a City statement): the “estimated project cost” on most ADU permits closely follows this per-square-foot table. That explains both the tight spread and the steady rise. It is a standardized valuation for setting fees, not a contract price.
What that number usually does not capture:
- Sales tax. In Washington, a contractor building for a homeowner collects retail sales tax on the full contract price (see below).
- Utility and agency charges billed outside SDCI: King County’s sewer capacity charge and, in some cases, Seattle Public Utilities (SPU) development charges.
- Design and engineering. Architect or designer fees, survey, structural and geotechnical work. See ADU design cost: architect vs. pre-approved plans.
- Site conditions. Long sewer or water runs, slopes, tree protection, retaining walls and electrical service upgrades. Our guide to hidden ADU costs and utility hookups walks through each one.
- Financing costs. Interest during construction, loan fees and appraisal.
Attention: Contractor websites that rank for “Seattle ADU cost” publish ranges that start well above the City’s valuation, often around $250,000 or more for a detached unit (for example, this builder guide). Those figures are marketing material and do not show how they were derived. Still, together with the list above, they suggest you should treat the permit median as a floor for the structure itself, not as an all-in price. That is our judgment, not a measured fact.
The fees and charges you can price today
Some parts of a Seattle ADU budget are set by public schedules. Our Seattle ADU fees guide covers them in depth. Here is the short version, with 2026 figures.
SDCI permit and plan review
SDCI does not publish a flat ADU fee. Fees come from the project valuation through the Fee Subtitle. The reliable way to price this line is SDCI’s fee guidance and estimator. We do not quote a dollar figure because the City publishes a formula, not an amount.
King County sewer capacity charge
King County charges $77.99 per residential customer equivalent per month in 2026. An attached or detached ADU counts as 0.59 of an equivalent, so it pays $46.01 a month for 15 years, billed quarterly.
Over the full term that is about $8,282. You can pay the balance early; ask the County for a payoff figure.
Seattle Public Utilities (SPU) charges
SPU’s Director’s Rule FIN-220.2, revision 5 (effective January 1, 2026) sets the utility’s development charges. The ones an ADU owner is most likely to meet:
| SPU charge (2026) | Published amount | When it is likely to apply |
|---|---|---|
| Water system development charge | $6,900 for a 3/4-inch service | A new water service, or a larger existing one |
| Wastewater system development charge | $2,600 for a 3/4-inch service | Same basis as water |
| Drainage system development charge | $1,225 per 1,000 sq ft of new hard surface | New roof, patio or paving |
| New 3/4-inch water service installation | $4,800 (non-arterial street) / $6,175 (arterial) | Only if SPU installs a new service |
| Side sewer permit, additional connection to an existing side sewer | $375 | Tying the ADU into a side sewer |
The rule ties the water and wastewater charges to the size of a new water service or the increase in size of an existing one. It does not mention ADUs.
Our reading: an ADU that shares the main house’s existing meter, without upsizing it, may not trigger the water and wastewater charges. A separate meter almost certainly would. That is an inference, so get SPU’s answer in writing. Our guide on whether ADUs need separate utility meters explains the trade-offs (it focuses on California, but the logic carries over).
Impact fees
We found no impact fee schedule that the City of Seattle applies to ADUs today. In November 2023 the City Council voted down the plan amendment that would have paved the way for a transportation impact fee. Confirm the current position with SDCI before you budget.
If a Washington city does charge impact fees, state law (RCW 36.70A.681) caps them for an ADU at 50% of the impact fees that would be imposed on the principal unit. The same statute bars owner-occupancy requirements and stops cities from capping ADU floor area below 1,000 sq ft. More in our Washington State ADU law guide and our explainer on ADU impact fees.
The line most budgets forget: sales tax
In Washington, building for a homeowner is a retail sale. Under WAC 458-20-170(4)(a), “prime contractors are required to collect from consumers the retail sales tax measured by the full contract price,” labor included.
The Department of Revenue lists Seattle’s combined rate at 10.55% from January 1, 2026 (6.5% state plus 4.05% local), after King County and Seattle each added a 0.1% local law enforcement programs tax (see the DOR rate notice). In practice:
- Every $100,000 of contract price adds $10,550 in sales tax.
- On a $200,000 contract, that is $21,100, more than the 15-year sewer capacity charge and SPU charges combined in most scenarios.
- Rates can change quarterly. Check your address with DOR’s tax rate lookup before you sign.
Practical tip: When you compare bids, ask each contractor whether the number includes sales tax. On a $150,000 job, a bid that leaves the tax out looks $15,825 cheaper on paper than an identical bid that includes it.
Pre-approved plans: cheaper, and faster?
Seattle’s pre-approved DADU program, ADUniverse, lets you build from a design the City has already reviewed. The permit data lets us test the two promises: lower cost and faster permits.
Cost. From 2023 to 2025, 151 of 1,112 detached ADU permits (about 14%) used a pre-approved plan. Their median estimated project cost was $173,602, against $184,515 for custom designs. That is about $11,000 less. Part of the gap may simply reflect smaller floor plans, because the dataset does not record square footage.
Time. We measured the days from the date SDCI accepted a complete application to the date it issued the permit:
| Year issued | Pre-approved plan: median days | Custom DADU: median days |
|---|---|---|
| 2023 | 55 (57 permits) | 162 (372 permits) |
| 2024 | 59 (65 permits) | 140 (370 permits) |
| 2025 | 88 (29 permits) | 160 (214 permits) |
Pre-approved plans clearly win on speed, by roughly two to three months. But they do not hit the “2–6 weeks” the City advertised when it launched the program in 2020. The City’s own 2023 ADU report measured it differently and reported an average of 80 days versus 133. Either way, a faster permit means fewer months of carrying costs on a construction loan.
Attention: Designers can charge a licensing fee to use their pre-approved plan. The ADUniverse 2.0 submission guide says the licensing fee structure was adjusted on May 29, 2026, so confirm the current fee for the plan you pick. For how pre-approved plans compare with hiring an architect, see ADU design cost: architect vs. pre-approved plans.
A worked example: 1,000 sq ft backyard cottage in Seattle
This is an illustration, not a quote. It prices only the lines that come from public schedules, for a hypothetical 1,000 sq ft wood-frame DADU on a non-arterial street, with 700 sq ft of new roof and paving.
| Line | Scenario A: shares the house’s water meter | Scenario B: new 3/4-inch water service | Basis |
|---|---|---|---|
| Structure at City valuation | $186,290 | $186,290 | 1,000 sq ft × $186.29 (DR 05-2025) |
| WA sales tax at 10.55% on that amount | $19,654 | $19,654 | WAC 458-20-170 |
| King County capacity charge (15 years) | $8,282 | $8,282 | 0.59 × $77.99 × 180 months |
| SPU water + wastewater SDC | $0 (if SPU agrees) | $9,500 | FIN-220.2 |
| SPU new water service installation | $0 | $4,800 | FIN-220.2 |
| SPU drainage SDC (700 sq ft) | $858 | $858 | 0.7 × $1,225 |
| Side sewer permit (additional connection) | $375 | $375 | FIN-220.2 |
| Subtotal of priced lines | $215,459 | $229,759 | |
| SDCI permit fees | Use SDCI’s estimator | Use SDCI’s estimator | Valuation-based |
| Design, survey, engineering | Not priced here | Not priced here | Get quotes |
| Site work, utility trenching, electrical upgrade | Not priced here | Not priced here | Get bids |
How to read it: before a single design or site-work dollar, the public-schedule lines already put this example at about $215,000–$230,000. The structure line is the City’s standardized valuation. If your contractor’s price is higher, which our reading of the evidence above suggests is common, the sales tax rises with it.
How to turn this into a real number for your lot
The permit data tells you where Seattle projects cluster. Your own number comes from a few calls and three bids. This is the order we would follow:
- Use SDCI’s fee guidance with a realistic valuation (your square footage × the current valuation table).
- Contact King County to confirm the capacity charge applies to your connection, and ask for the lump-sum payoff figure.
- Ask SPU development services whether your ADU needs a new water service, and whether the water, wastewater and drainage charges apply. Get the answer in writing.
- Collect three line-item bids with the same scope. Make sure each one states whether sales tax is included, and what it leaves out (design, permits, utility trenching, landscaping, appliances).
- Add a contingency. We do not have Seattle data on overruns, so pick a percentage you are comfortable with and tell your lender. Our ADU budget breakdown shows how to structure one.
Key fact: Lenders size construction loans on documented costs. A bid package that separates structure, sales tax, utility charges and soft costs is easier to underwrite than a single per-square-foot number. See ADU construction loans and how to finance an ADU.
What to confirm before you rely on these figures
- Ask SDCI how it set the estimated project cost on recent ADU permits near you; the tight clustering points to the valuation table.
- Ask SPU in writing whether water and wastewater development charges apply if your ADU shares the existing meter.
- Ask the designer for the current ADUniverse licensing fee, which changed on May 29, 2026.
- Check SDCI’s codes page for a valuation table newer than Director’s Rule 05-2025.
- Ask SDCI whether any impact fee applies to ADUs today.
Frequently asked questions
How much does a detached ADU cost in Seattle?
Seattle’s permit records show a median estimated project cost of $196,326 for DADU permits issued in 2025. That figure tracks the City’s valuation table (about $186 per square foot for wood-frame construction). Washington sales tax (10.55% in Seattle), King County’s sewer capacity charge (about $8,282 over 15 years) and possible SPU charges come on top.
Is an attached ADU cheaper than a backyard cottage?
Usually, yes. For permits issued in 2023–2025, the median estimated cost was $123,069 for attached ADUs and $182,874 for detached ones. Attached projects vary much more: the middle half ranged from $50,000 to about $216,000, because a basement with ceiling height and a bathroom rough-in is a very different job from carving a unit out of finished living space.
Do pre-approved plans save money in Seattle?
Some. From 2023 to 2025, pre-approved detached ADUs had a median estimated cost of $173,602, about $11,000 lower than custom designs, and their permits were issued roughly two to three months faster by median days. Part of the cost gap may reflect smaller floor plans. Check the designer’s licensing fee, which changed in May 2026, before you choose a plan.
Does Seattle charge impact fees on ADUs?
We found no Seattle impact fee schedule that applies to ADUs, and the Council rejected a step toward a transportation impact fee in 2023. Confirm with SDCI. If a Washington city does charge impact fees, state law caps an ADU’s fees at 50% of what the main house would pay.
Can rental income help me qualify for the loan?
Sometimes. It depends on the loan program and how the lender documents projected rent. Our guide on ADU rental income and loan qualification explains the rules. If you have equity, compare a HELOC for an ADU with a construction loan.
The bottom line
Seattle’s permit records put the median estimated cost of a detached ADU at $196,326 for 2025 permits and of an attached ADU at $123,069 for 2023–2025, but those figures track the City’s valuation table, not contract prices. Washington’s 10.55% sales tax in Seattle, King County’s sewer capacity charge of about $8,282 over 15 years and possible SPU charges come on top.
Your next step is to price the public-schedule lines for your lot as in the worked example, then collect three line-item bids that state whether sales tax is included. Compare the charges in Seattle ADU fees and the state rules in Washington State ADU law.
General information, not financial, tax or legal advice. Fees, tax rates and valuation tables change; confirm with SDCI, SPU, King County and the Washington Department of Revenue.
Methodology. Data: City of Seattle, Building Permits (data.seattle.gov, dataset 76t5-zqzr), downloaded October 22, 2026. Filter: dwelling unit type exactly “Accessory Dwelling Detached” or “Accessory Dwelling Attached”, issued January 1, 2021 to December 31, 2025, with an estimated project cost above $1,000. Result: 1,461 permits for 2023–2025 (1,112 detached, 349 attached) plus 633 detached permits for 2021–2022 used only in the year table. Medians and percentiles (linear interpolation) are by issue year. Permit time = issue date minus application date, where the dataset defines application date as the date SDCI accepted the application as a complete submittal. Pre-approved status comes from the dataset’s “standardplan” field.
Sources: City of Seattle open data: Building Permits · SDCI Director’s Rule 05-2025, Building Valuation Data · SDCI: How much will your permit cost? · SDCI Fee Subtitle · King County: Capacity charge · SPU Director’s Rule FIN-220.2 (rev. 5) · RCW 36.70A.681 · WAC 458-20-170 · WA DOR: King County local sales tax rates, Q1 2026 · OPCD ADU Report 2023 · ADUniverse 2.0 submission guide · City of Seattle, pre-approved DADU launch (2020) · Planetizen: Seattle Council rejects transportation impact fee (2023)
Cover photo: “American Craftsman-style home at 1903 NE Naomi Place Seattle” by Guywelch2000, CC BY 4.0, cropped, via Wikimedia Commons.
Questions or a correction?
If something in this guide is unclear, out of date or does not match what your city told you, send me a message. Include the source if you have one.