Do I Need a Permit to Build an ADU in Seattle, WA?

ADU-specific overview

What makes this different from a simple accessory structure

Pulled from the city's ADU guidance and linked code references.

Permit type: Residential building permit (new construction or addition) through Seattle DCI. AADU and DADU both follow the standard residential permit path.
Maximum size: DADU: 1,000 square feet of gross floor area, maximum 24 feet in height under SMC 23.44.041. AADU size is limited to no more than the gross floor area of the primary unit or 1,000 square feet, whichever is less.
Setbacks: DADU: minimum 5 feet from rear lot line and 5 feet from each side lot line per SMC 23.44.041. Front setback must meet the standard zone requirement. No minimum separation from the primary dwelling is required.
Owner occupancy: No owner-occupancy requirement. Washington HB 1337 (2023) prohibited cities from requiring the property owner to live on site as a condition of ADU construction or rental.
Parking: No additional off-street parking is required for an ADU under Seattle zoning. Existing required parking serving the primary dwelling must not be eliminated.
Utilities: DADUs may share utilities with the primary dwelling or have separate connections depending on design. Separate sewer connection or utility tap triggers Seattle Public Utilities review and fees. Confirm with DCI at pre-application if separate metering is planned.
JADU distinction: Seattle does not use the California-style JADU designation. ADUs within the existing primary structure are called Attached ADUs (AADUs) and follow the same SMC 23.44.041 standards rather than a separate junior-unit track.

Washington HB 1337 (2023) is a statewide mandate: cities must allow at least two ADUs per single-family lot and may not require owner occupancy.
Seattle allows one AADU plus one DADU (or two DADUs) on a qualifying lot under current rules.
Seattle tip sheets for ADUs are available on the SDCI website and cover submittal requirements and design standards in detail.

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.

Documents

Required documents

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.

Timeline

Typical timing

Plan review: 10–20 business days for standard residential plan review; over-the-counter review may be available for simple projects
Total cycle: 6–16 weeks typical from submittal to permit issuance; total project time including construction and final inspection varies

Correction rounds extend review time. Seattle DCI tracks current residential plan review turnaround times on its website. Expedited review is available for an additional fee.

Process

How the permit process works

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.
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.
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.
Submit through the Seattle Services Portal File the building permit application online at seattleservicesportal.seattle.gov. Upload all required plan documents. Pay the intake fee at submittal.
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.
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.
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.

Code basis

What Seattle reviews against

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 Seattle permit portal

If you skip the permit

What can go wrong

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.

Sources

Official links and freshness

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