What's Required in Seattle
- Every AADU or DADU needs an SDCI construction addition/alteration permit with plan review; legalizing an existing unit needs a permit to establish use.
- Permit and plan review fees are each 100 percent of the Table D-1 index on the valuation-table value ($186.29 per sq ft for Type VB R-3 construction), plus 5 percent technology fee and the $6.50 state surcharge; about 75 percent is due at intake.
- The unit must meet current Seattle residential, building, mechanical, electrical, energy, land use, ECA and shoreline codes; a DADU is not treated as an accessory structure under the building codes.
- DADU limits in NR zones (Tip 116B): 3,200 sq ft minimum lot, 1,000 sq ft maximum unit, 40 percent of the required rear yard, 5 ft setbacks (0 ft on an alley), no shoreline districts.
- No parking is required for an ADU, but existing required parking cannot be removed without replacement.
- Separate SDCI electrical permit (contractor applies unless the owner does all the work) and Public Health plumbing permit ($137 plus $27 per fixture); a side sewer permit may be required.
- Sign the King County sewer capacity charge acknowledgment; the county bills quarterly for 15 years after the connection.
- Pre-approved standard plans are eligible only in NR zones with no ECA and under 750 sq ft of ground disturbance.
Do You Need a Permit for an ADU in Seattle?
Yes, always, with full plan review. SDCI's ADU page says a unit is not legal unless established through a permit: adding a unit inside the house (an AADU) or building a backyard cottage (a DADU) takes a construction addition/alteration permit, and legalizing an existing unit takes a permit to establish use. Tip 101 confirms plan review for every AADU and DADU. The fee is value-based; SDCI's own 2026 example is a 500 sq ft DADU valued at $93,145 paying $3,453 in plan review and permit fees, up 18.7 percent from 2025. Seattle allows two ADUs per lot with a principal unit, including two DADUs, with no parking requirement. Pre-approved DADU plans get a permit in 2 to 6 weeks in most cases.
Seattle ADU Permit Rules by Scenario
| What you are doing | City filing | Rule |
|---|---|---|
| New detached ADU (backyard cottage), 500 sq ft | Addition/alteration permit with plan review: $3,453 (SDCI example) plus 5 percent and $6.50 | SDCI 2026 Fee Summary; ADU page |
| DADU built from a pre-approved standard plan in an NR zone, no ECA, under 750 sq ft of ground disturbance | Same permit on the expedited track, 2-6 weeks in most cases | SDCI ADU page; Portal help article |
| AADU inside the existing house (basement or upper-floor conversion) | Addition/alteration permit with plan review, value-based | SDCI ADU page; Tip 116A |
| Legalizing a unit that already exists without a permit | Construction permit to establish use | SDCI ADU page |
| Wiring the unit (new feeder, subpanel, circuits) | SDCI electrical permit; contractor applies unless the owner does all the work | Tip 116B; Table D-15 |
| Plumbing for kitchen and bath | Public Health plumbing permit: $137 plus $27 per fixture | Tip 116B; PHSKC fee schedule |
| Connecting to the sewer | King County capacity charge billed quarterly for 15 years; side sewer permit may be required | Tip 116B; ADU page |
How Much Does an ADU Permit Cost in Seattle?
SDCI prices the construction permit from Table D-1 of the 2026 Fee Subtitle on value, set from its building valuation data: Director's Rule 5-2025 values one- and two-family residential construction (Group R-3) at $186.29 per square foot for Type VB wood frame. Permit fee and plan review fee are each 100 percent of the Development Fee Index, plus a 5 percent technology fee and the $6.50 state surcharge; about 75 percent is due at intake.
SDCI's own example: 500 sq ft x $186.29 = $93,145. Table D-1 gives $1,471.50 for the first $75,000 plus $1.40 per $100 above it (182 x $1.40 = $254.80), an index of $1,726.30; doubled for permit plus plan review that is $3,452.60, which SDCI rounds to $3,453; with technology fee and state surcharge, $3,631.73. A 1,000 sq ft DADU, the NR-zone maximum, is $186,290 of value: $2,402.75 for the first $175,000 plus 12 x $7.75 = $93, index $2,495.75, doubled $4,991.50, all in $5,247.58. An AADU conversion valued at $60,000 (labor and materials) has an index of $1,246.50 and totals $2,624.15. Then budget the SDCI electrical permit (a 60 to 200 amp feeder is $146 plus $26.28 per small circuit and $55.48 administration), the Public Health plumbing permit ($137 plus $27 per fixture, about $326 for seven), a possible side sewer permit, and the King County capacity charge, billed quarterly for 15 years.
Size, Height and Placement Rules
Tip 116B (2023) sets the DADU limits in Neighborhood Residential zones: a lot of at least 3,200 sq ft, a unit of no more than 1,000 sq ft gross (excluding garages, exterior-access storage, small porches and below-grade area), accessory structures covering no more than 40 percent of the required rear yard, and no parking requirement, though existing required parking cannot simply be removed. Tip 116A caps an AADU at 1,000 sq ft unless that part of the house existed on December 31, 2017. SDCI's 2025 zoning summary lists ADU setbacks of 5 ft (0 ft on an alley) and a 32 ft zone height limit. DADUs are barred in shoreline districts. Ordinance 127376 rewrote SMC 23.44 in 2025 and the ADU standards now sit in SMC 23.42.022, so confirm the Tip figures with SDCI's free land use coaching. Owner occupancy is not among either Tip's criteria, and SDCI publishes a form releasing older owner-occupancy covenants.
The Process
File a Building & Land Use Pre-Application in the Seattle Services Portal with a site plan; SDCI assigns a record number and may require a pre-application site visit. Intake appointments can be 2 to 4 months out, so upload the full plan set (site plan, floor plans of both units, elevations, structural plans, energy calculations, capacity charge form) early to be pulled forward. After intake you pay the fees and respond to corrections; a tenant may occupy the unit only after the final inspection.
How to Get a ADU Permit in Seattle
Check feasibility
Use the ADUniverse search tool and SDCI's GIS map to confirm the zone, lot size, ECA status and shoreline overlay, then decide between a custom design and a pre-approved DADU plan.
Submit the pre-application
File a Building and Land Use Pre-Application in the Seattle Services Portal with a site plan and legal description; SDCI assigns a record number and may schedule a pre-application site visit.
Prepare the full plan set
Site plan, floor plans of both units, elevations, structural plans, energy calculations, the Statement of Financial Responsibility and the King County capacity charge form; upload everything early to move your intake appointment up.
Pay fees and clear corrections
About 75 percent of the value-based fee is due at intake; respond to correction letters in the portal until the plans are approved and the permit is issued electronically.
Build, inspect, then occupy
Pull the electrical permit and the Public Health plumbing permit, schedule inspections through the portal or (206) 684-8900, and rent the unit only after the final inspection is approved.