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. State law bars a city maximum below 1,000 sq ft of gross floor area, except on lots with critical areas (RCW 36.70A.681).
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 $6.50 + 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