Short Answer
Yes, construction permitSeattle requires a construction permit for an attached ADU (AADU) or detached ADU (DADU), whether new or created by addition or alteration. Legalizing an existing unit needs a permit to establish use. A lot can have at most 2 ADUs, and parking, street improvement and Mandatory Housing Affordability requirements do not apply.
Source: Seattle SDCI: Accessory Dwelling Units (linked under Sources below)
Seattle ADU Permits at a Glance
- ADU permits, last 12 months
- 482
- Median application to issue
- 187 days
- Slowest quarter took at least
- 303 days
- New detached ADUs
- 329
Of the 7 cities we track for ADUs, Seattle ranks 5 of 7 by median wait. The median declared job value on these permits was $236,540.
Who Can Apply and Do the Work
SDCI reviews ADU permits. Pre-approved DADU plans can get expedited review when the lot is in an NR1, NR2 or NR3 zone, has no environmentally critical area, and the project disturbs less than 750 square feet of ground.
What You Need Before the Permit Is Issued
- Prepare a site plan.
- Complete the King County sewer capacity charge form.
- Submit a rat abatement form if a garage or shed will be removed.
- Check whether a pre-approved DADU plan fits the lot.
What Else to Know in Seattle
SDCI Tip 316 rules out the faster Subject-to-Field-Inspection permit for establishing, constructing or converting an ADU or DADU, so these always go through plan review. Plumbing permits come separately from Public Health – Seattle & King County.
Seattle ADU Permit Costs
Seattle does not publish a flat ADU permit fee; plan check and permit fees are calculated from the project valuation when you apply, so we don't quote one. For scale, the median declared value of ADU work on recent permits was $236,540.
What Seattle's ADU Permit Data Shows
| Type of ADU | Permits | Share |
|---|---|---|
| Conversion of a garage, basement or other existing space | 37 | 8% |
| New detached ADU | 329 | 68% |
| Attached ADU | 116 | 24% |
| Junior ADU | 0 | 0% |
| Not specified in the description | 0 | 0% |
ADU Permits Issued per Month
Types are classified from each permit's description. Building permits whose description mentions an ADU, AADU or DADU. Data from Seattle Building Permits, October 30, 2025 to October 30, 2026.
Where ADUs Are Being Permitted
| ZIP code | Permits (12 mo) |
|---|---|
| 98115 | 63 |
| 98117 | 47 |
| 98125 | 42 |
| 98108 | 31 |
| 98106 | 31 |
| 98103 | 30 |
Recent ADU Permits
| Issued | Work as described by the city |
|---|---|
| 2026-10-28 | Construct new one family dwelling (East DADU) per plan. (Allow new detached accessory dwelling units to existi |
| 2026-10-22 | Construct addition and alterations to existing west single family dwelling per plan. (Allow housing for one de |
| 2026-10-22 | Establish use as single-family with attached accessory dwelling unit per land use code. Construct two-family d |
| 2026-10-22 | Construct alterations to convert detached garage into east single family dwelling unit per plan. (Allow housin |
| 2026-10-21 | Allow new detached accessory dwelling unit to existing single family use per land use code. Construct west one |
| 2026-10-19 | Construct new one-family dwelling (SFR 3) per plan. (Establish use as five single family residences one with a |
How to Get an ADU Permit in Seattle
- Confirm what your lot allows. Zoning, existing unit count, hillside, fire and historic status decide which ADU types and how many you can add.
- Choose the program and plans. Decide between the State program and the Local program; they cannot be combined.
- Complete the prerequisites listed above before you file.
- File the building permit through PermitSF, over the counter or for in-house review.
- Build, pass inspections and get final sign-off before anyone moves in.
Frequently Asked Questions
Do I need a permit to build an ADU in Seattle?
Seattle requires a construction permit for an attached ADU (AADU) or detached ADU (DADU), whether new or created by addition or alteration. Legalizing an existing unit needs a permit to establish use. A lot can have at most 2 ADUs, and parking, street improvement and Mandatory Housing Affordability requirements do not apply.
How long does an ADU permit take in Seattle?
For the 481 ADU permits issued between October 30, 2025 and October 30, 2026 with both dates published, the median time from application to issue was 187 days, and a quarter took 303 days or longer, according to the city's open data.
How many ADU permits does Seattle issue?
482 in the 12 months to October 30, 2026. About 8% converted existing space such as a garage, 68% were new detached units, 24% were attached and 0% were junior ADUs, based on the permit descriptions.
How many ADUs can a Seattle lot have?
Up to 2.
Does a Seattle ADU need parking?
No. Parking, street improvement and MHA requirements do not apply to ADUs.
Related Permit Guides
- Seattle Building Permits
- ADU Permit Requirements: What Homeowners Need to Know
- Los Angeles ADU Permit Requirements
- San Francisco ADU Permit Requirements
- San Diego ADU Permit Requirements
- Chicago ADU Permit Requirements
- Austin ADU Permit Requirements
- New York ADU Permit Requirements
Sources
Official sources checked on . Rules change; confirm with the department before you start work.
- Seattle SDCI: Accessory Dwelling Units (seattle.gov)
- SDCI Tip 316: Subject-to-Field-Inspection Permits (seattle.gov)
- Seattle Building Permits (cos-data.seattle.gov)