The short answer: Seattle ADU fees include no ADU-specific permit fee: the Seattle Department of Construction and Inspections (SDCI) bases its fees on project valuation. The charge many owners miss is King County’s sewer capacity charge. An ADU counts as 0.59 of a residential customer equivalent, and at the 2026 rate of $77.99 per equivalent per month that is $46.01 a month for 15 years, about $8,282 if paid over the full term. In addition, Seattle Public Utilities (SPU) also publishes separate water, wastewater and drainage development charges, and whether they apply depends on your connections. We found no neutral data on Seattle ADU construction costs, so this guide covers Seattle ADU fees as the City, SPU and the County publish them.
Last verified: October 5, 2026 ยท Written by the ADU Scope editorial team
Attention: All figures below come from City of Seattle, SPU and King County pages. Those pages leave several questions open, and each needs a written answer from the agency: whether the County charge applies to a detached ADU tied into an existing side sewer, and whether SPU’s development charges apply to an ADU that shares the main house’s meter. We flag each one where it comes up.
Seattle ADU Fees From SDCI: Permit and Plan Review
First, Seattle’s 2026 Fee Subtitle, effective January 1, 2026, sets development fees from project valuation, expressed as percentages of a Development Fee Index, rather than as a flat amount. Notably, SDCI’s fee pages show no separate ADU fee. Four details help you budget:
- Estimator: SDCI publishes an Excel fee estimator, with a 2026 version, that you can fill in with your project’s valuation.
- Technology fee: the subtitle adds a charge of five percent of the fees required under the development chapters.
- Hourly rates: the subtitle lists an SDCI hourly rate of $292 for most work and a Land Use hourly rate of $551. Therefore, ask SDCI when hourly billing applies to your project.
- Payment timing: for building permits that need plan review, you owe 75% of the fees when SDCI accepts the application and the rest before it issues the permit.
We do not quote a dollar total for a Seattle ADU permit, because the City publishes a formula and an estimator rather than a fixed ADU amount.
King County Sewer Charge in Seattle ADU Fees
First, King County’s Wastewater Treatment Division bills a capacity charge to owners who connect to the sewer system. The County bills it monthly for 15 years, and you may pay the remaining balance in one lump sum at a discount at any time during that period. However, the County’s page does not state the discount.
| Item | Figure | Source |
|---|---|---|
| Rate for connections on or after January 1, 2026 | $77.99 per residential customer equivalent per month | King County |
| Detached or attached ADU | 0.59 residential customer equivalent | SDCI/King County form |
| Monthly charge for an ADU | 0.59 x $77.99 = $46.01 | Our calculation |
| Total over 15 years, undiscounted | $46.01 x 180 months = about $8,282 | Our calculation |
Specifically, the charge applies, per the SDCI form, to all new sewer connections, reconnections or changes of use of existing connections. In addition, SDCI reports every ADU permit to King County, and applicants sign an acknowledgment form. The form says the County may give credit for a demolition in some circumstances.
Key fact: You pay the charge over 15 years, which makes it easy to overlook in a first budget. However, we could not find a page that states outright whether a detached ADU tied into the existing side sewer pays it, although the SDCI form lists the ADU rate and the County describes the charge as applying to additions and changes of use. Therefore, get the answer from King County in writing before you rely on a figure of $0 or $8,282.
SPU Development Charges in Seattle ADU Fees
SPU’s Director’s Rule FIN-220.2 (revision 5), effective January 1, 2026, sets these development charges:
| Charge | Published rate |
|---|---|
| Water | $6,900 per unit rate multiplied by a meter-size factor; a 3/4-inch meter is $6,900 |
| Wastewater | $2,600 for a 3/4-inch connection |
| Drainage | $1,225 per 1,000 sq ft of new hard surface |
The material we reviewed does not address ADUs specifically, and we could not confirm whether an ADU that shares the main house’s meter triggers these charges. Instead, treat the rates as the published schedule and ask SPU’s development services office how they apply to your connections. Still, a drainage charge based on new hard surface does suggest that paving and roof area on your site matter.
Seattle ADU Fees: Are There Impact Fees?
First, the City’s Office of Planning and Community Development states that Seattle currently does not charge any impact fees. The page is undated and does not mention ADUs, so please confirm with the City. Beyond that, where a Washington city does charge impact fees, state law (RCW 36.70A.681) provides that it may not assess impact fees on an ADU that are greater than 50% of the impact fees imposed on the principal unit, and the same statute bars owner-occupancy requirements. For more detail, our guide to Washington State ADU law covers the framework.
Pre-Approved Plans and Review Time
Seattle’s pre-approved ADU program goes by the name ADUniverse. The City says it can issue permits for pre-approved designs in as little as two to six weeks on most sites, and that permit fees drop because the City needs less review. However, it gives no dollar figure for the reduction, and designers may charge a licensing fee of up to $1,000 in the current gallery. The City’s submission guide for the next round schedules the new gallery launch for November 2026, so check which designs are live when you apply. See ADU design cost: architect vs. pre-approved plans.
In contrast, for other projects, SDCI says it typically completes the initial plan review of simple and medium-complexity projects within two to four weeks after it accepts the application, and gives a target of eight to twelve weeks for complex projects. However, those figures are not specific to ADUs. Also, we could not retrieve SDCI’s published intake-appointment wait times, so ask when you book.
A Map of Seattle ADU Fees
| Charge | Who bills it | Basis | What to confirm |
|---|---|---|---|
| Permit and plan review | SDCI | Project valuation, plus 5% technology fee | Estimator result; when hourly rates apply |
| Sewer capacity charge | King County | 0.59 customer equivalent x $77.99 per month for 15 years | Whether it applies to your connection; early-payment discount |
| Water, wastewater and drainage development charges | SPU | Meter size, connection and hard surface | Whether they apply to an ADU on an existing meter |
| Impact fees | City of Seattle | City says it charges none | Current status; state cap of 50% if charged |
| Design licensing fee (pre-approved plans) | Plan designer | Up to $1,000 in the current gallery | Which catalog is live |
What Does It Cost to Build?
We did not find neutral data on Seattle ADU construction costs from the City or the state, and we do not quote builder ranges for Seattle because no primary source backs them. Meanwhile, the City’s Office of Planning and Community Development publishes annual ADU reports that are worth checking for local data. For a method that works in any city, see what an ADU costs in California and our ADU budget breakdown: get three written bids with line items and the same inclusions, and ask each bidder what the price leaves out.
Frequently Asked Questions
How are Seattle ADU fees for permits set?
SDCI bases its fees on valuation and sets no ADU-specific amount. Therefore, use SDCI’s fee estimator with your project valuation, and add the 5% technology fee.
What is the King County sewer charge for an ADU?
For example, at the 2026 rate of $77.99 per equivalent per month and an ADU rate of 0.59 equivalent, it is $46.01 per month for 15 years, about $8,282 undiscounted. However, confirm that it applies to your connection.
Does Seattle charge ADU impact fees?
The City says it does not currently charge impact fees. In addition, state law caps ADU impact fees at 50% of the principal unit’s where a city does charge them.
Is a pre-approved plan faster?
Yes, because the City says two to six weeks on most sites for pre-approved designs.
Next Steps
- Washington State ADU Law: What Homeowners Can Build
- Hidden ADU Costs and Utility Hookups
- ADU Design Cost: Architect vs. Pre-Approved Plans
- ADU Cost in Portland: SDCs, the 10-Year Waiver and Permit Fees
General information, not financial or legal advice. Fees and rates change; confirm with SDCI, SPU and King County.
Sources: SDCI: 2026 Fee Subtitle ยท SDCI: How much will your permit cost? ยท King County: Sewer capacity charge ยท SDCI: King County capacity charge form ยท SPU: Director’s Rule FIN-220.2 ยท Seattle OPCD: Impact fees ยท RCW 36.70A.681 ยท City of Seattle: ADUniverse gallery ยท SDCI: Review status and target due dates