Permit Required? Required
Typical Fee Range $3,500–$8,500 Includes $292/hr plan review and King County sewer capacity charges; fees increased by 18% in 2026.
Fee Method Valuation-based
Apply Online Apply at Seattle Department of Construction and Inspections →

What's Required in Seattle

  • Up to two ADUs allowed per lot (e.g., one attached and one detached)
  • Maximum size of 1,000 square feet for a detached unit (DADU)
  • Maximum height of 32 feet allowed in Neighborhood Residential zones (as of 2025/2026)
  • Must maintain 5-foot setbacks from side and rear property lines
  • No additional off-street parking is required for ADUs citywide
  • Owner-occupancy of either the main house or ADU is no longer required in Seattle

How Much Does an ADU Permit Cost in Seattle?

Budget $3,500–$8,500 for the Seattle permit itself (valuation-based), issued by the Seattle Department of Construction and Inspections — but treat that as the entry fee, not the total. ADUs are full dwelling units, so the permit stack usually includes building, electrical, plumbing, and mechanical trade permits, plus plan review. One local note: includes $292/hr plan review and King County sewer capacity charges; fees increased by 18% in 2026. Get an itemized fee estimate from the department before finalizing your budget; the spread between a simple conversion and a new detached build is wide.

ADU Size Limits and Zoning Rules in Seattle

Before drawing plans, check the two gates that sink most ADU projects: size and zoning. On record here, the size ceiling is 1,000 square feet for a detached unit (DADU). Other recorded conditions: no additional off-street parking is required for ADUs citywide; owner-occupancy of either the main house or ADU is no longer required in Seattle. Setbacks, height limits, and utility connections layer on top of those rules. The Seattle Department of Construction and Inspections can confirm what your specific lot allows — worth doing before you pay for architectural drawings rather than after.

How to Get a ADU Permit in Seattle

1

Step 1: Feasibility Study

Verify lot size (min 3,200 sq ft) and zoning eligibility via the ADUniverse tool.

2

Step 2: Pre-Application

Submit a Building & Land Use Pre-Application to verify the legal address and description.

3

Step 3: Intake Appointment

Schedule a virtual intake appointment to submit structural, drainage, and energy plans.

4

Step 4: Fee Payment

Pay the plan review and King County sewer capacity deposit via the portal.

5

Step 5: Inspection Series

Schedule Foundation, Framing, Rough MEP, Insulation, and Final inspections.

Frequently Asked Questions

Yes, recent code changes allow up to two ADUs per lot, which can include two detached units.
The maximum height is 32 feet in standard residential zones to allow for two-story units.
Yes, the city offers several 'Pre-Approved DADU' designs that bypass standard plan review times.
On record, the ceiling is 1,000 square feet — with the exact limit depending on ADU type and lot. Confirm your configuration with the Seattle Department of Construction and Inspections.