Fees
Estimated city fees
Baseline for a simple permitted adu: Seattle DCI permit and plan review fees are valuation-based. A detached ADU valued at $150,000–$250,000 typically generates $2,000–$5,000+ in combined permit and plan review fees. Verify current amounts at seattle.gov/sdci.
| Fee | Amount | Notes |
|---|---|---|
| Permit fee | Valuation-based | Seattle DCI charges permit fees based on the declared construction valuation of the project. Use the Seattle DCI fee estimator or contact SDCI directly for a current estimate. |
| Plan review fee | Valuation-based | A plan review fee is charged in addition to the permit fee for projects requiring plan review. Seattle charges this as a percentage of the construction valuation. |
| Washington State Building Code Council surcharge | $6.50 per permit | A state surcharge applies to each building permit issued in Washington. |
| Utility-connection and side-sewer fees | Project-specific | If the DADU requires a new sewer connection or separate utility service, additional Seattle Public Utilities or Seattle City Light fees apply. |
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Conditions
The rules that apply
- A building permit is required to construct any Accessory Dwelling Unit (ADU) in Seattle, whether attached (AADU) or detached (DADU).
- Seattle Municipal Code 23.44.041 governs ADUs in single-family residential zones (SF 5000, SF 7200, SF 9600, and RSL). Both attached and detached ADU types are allowed.
- Washington HB 1337 (effective July 2023, applicable January 2024) requires cities to allow at least two ADUs per lot on single-family-zoned parcels as a matter of state law.
- Seattle allows one attached ADU (AADU) and one detached ADU (DADU) on the same lot, up to two ADUs total per lot under the state mandate.
- A DADU must not exceed 1,000 square feet of gross floor area. Height is limited to 24 feet in most cases under SMC 23.44.041.
- Setback requirements under SMC 23.44.041: a DADU must maintain at least 5 feet from rear and side lot lines. The standard front setback for the zone still applies.
- HB 1337 eliminated owner-occupancy requirements statewide. Seattle no longer requires the property owner to live on site as a condition of building or renting an ADU.
- No additional off-street parking is required for a new ADU in Seattle. Existing required parking for the primary dwelling may not be removed.
Documents
What you'll need to file
- Completed building permit application submitted through the Seattle Services Portal.
- Site plan showing lot lines, existing dwelling, proposed ADU footprint, setback dimensions, driveway, trees, and utility locations.
- Architectural plans including floor plans, elevations, roof plan, and section details for the ADU.
- Structural plans or engineering calculations if the design requires structural members beyond prescriptive code.
- Energy compliance documentation (Washington State Energy Code compliance worksheet or modeling).
- Stormwater or drainage checklist if the project adds impervious surface beyond the city threshold.
- If a contractor is pulling the permit, the Washington State contractor registration number and authorization from the property owner.
Process
How the permit process works
Sequential — each step gates the next.
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Confirm the lot qualifies Verify the parcel is in a single-family zone (SF 5000, SF 7200, SF 9600, or RSL) using Seattle's online zoning map. Check existing ADU count on the lot — HB 1337 allows up to two ADUs per lot.
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Determine the ADU type (AADU or DADU) Decide whether the project is an attached ADU (created within or added to the primary structure) or a detached ADU (a separate structure). The permit path is the same but plan requirements differ.
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Prepare permit documents Assemble site plan, architectural drawings, structural details (if needed), energy code compliance documents, and any required stormwater materials. Seattle's Tip Sheet 104A (ADU) covers submittal requirements.
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Submit through the Seattle Services Portal File the building permit application online at cosaccela.seattle.gov. Upload all required plan documents. Pay the intake fee at submittal.
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Respond to plan review comments Seattle DCI reviewers will issue correction notices if additional information or plan changes are needed. Respond promptly to each correction cycle to move the application forward.
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Pay fees and issue the permit After plan approval, pay the remaining permit fees. The permit is issued and construction may begin once it is posted at the job site.
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Complete inspections and close out Schedule required inspections (foundation, framing, insulation, final) through the Seattle Services Portal. After all inspections pass, Seattle DCI closes the permit and the ADU is authorized for occupancy.
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Tools & materials
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Provenance
Code basis & official sources
Last verified 2026-04-29.
Seattle Municipal Code 23.44.041 (ADU standards), Seattle Building Code, Washington State Energy Code, and Washington HB 1337 (2023) statewide ADU reform.
Residential permit page: https://www.seattle.gov/sdci/permits/permits-we-issue/building-permits/accessory-dwelling-units
If you skip the permit
- Seattle DCI can issue a Stop Work Order and require the structure to be brought to permitted status or removed.
- An unpermitted ADU does not have a Certificate of Occupancy, making it unlawful to rent or occupy.
- After-the-fact permitting (permit for already-built work) is more expensive and may require opening completed walls or demolishing non-compliant work.
- Unpermitted ADUs can trigger code enforcement complaints, daily fines, and liens under the Seattle Municipal Code.
- Title, insurance, and financing issues arise when a property is sold or refinanced with an unpermitted dwelling unit.
- Washington requires a registered contractor for permitted work performed by a contractor; using an unregistered contractor adds legal exposure under RCW 18.27.
FAQ
Common Seattle adu permit questions
Do I need a permit for a detached ADU (DADU) in Seattle?
Yes. Any detached ADU requires a residential building permit from Seattle DCI. This includes new detached structures built as ADUs and conversions of existing detached garages or outbuildings to ADU use.
How many ADUs can I build on my Seattle property?
Up to two. Washington HB 1337 (effective 2024) requires Seattle to allow at least two ADUs per single-family lot — typically one attached ADU (AADU) and one detached ADU (DADU), or two DADUs on a qualifying lot.
Does Seattle require owner occupancy for an ADU?
No. Washington HB 1337 (2023) removed the authority for cities to require owner occupancy as a condition of building or renting an ADU. Seattle no longer has this requirement.
What is the maximum size for a detached ADU in Seattle?
Under SMC 23.44.041, a DADU may not exceed 1,000 square feet of gross floor area and is limited to 24 feet in height in most single-family zones.
Does building an ADU in Seattle require additional parking?
No. Seattle zoning does not require additional off-street parking for a new ADU. Existing required parking for the primary dwelling must not be removed.
How long does it take to get an ADU permit in Seattle?
Standard residential plan review at Seattle DCI typically takes 10–20 business days per round. Total time from submittal to permit issuance is commonly 6–16 weeks, longer if correction rounds are needed. Expedited review is available for an additional fee.
What does WA HB 1337 mean for Seattle ADU owners?
HB 1337 (signed 2023, effective 2024) is a statewide law that requires Washington cities to allow at least two ADUs per single-family lot and prohibits owner-occupancy requirements. It overrides any stricter local rules and is the legal foundation for Seattle's current two-ADU allowance.
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Disclaimer: Informational only — not legal advice. Rules change; verify with Seattle permitting staff before you build.