Detached ADU (DADU) Permits in City of Mercer Island
A standalone backyard cottage with its own foundation, roof, and exterior. Maximum flexibility on layout; highest construction cost. HB 1337 floor: 1,000 sf gross allowed everywhere.
Drops the placement canvas on your parcel for detached adu (dadu) scope. Live setback + coverage + easement checks; packet generation when you’re ready.
Permit fees in City of Mercer Island
Fee schedule not yet aggregated — check the City of Mercer Island fee schedule directly.
City of Mercer Island fee schedulePhase A Land Use fee per MI Fee Schedule (Type II Land Use Review line). Phase B Building permit fee is valuation-based: ~$2,100 building + ~$1,365 plan review (65%) + WA BCC $4.50 + per-fixture plumbing/mech fees at $280k valuation. Impact fees (school, transportation, parks) ~$5,000-$15,000 BEFORE HB 1337's 50% ADU cap — confirm MI has updated its schedule.
Review timeline
Typical: 30–90 business days
What a DADU permit packet contains
- A1.1 — Site plan with rotated footprint, setbacks, coverage
- A2.1 — Floor plan with rooms, openings, egress
- A3.1 — Elevations (all four faces, dim strings)
- A4.1 / A4.2 — Foundation + framing plans
- A5.1 — Wall section with WSEC R-values
- A6.0 — Glazing schedule (NFRC U-factor + SHGC)
- Filled application forms (city-specific)
- Manual J energy compliance report