Attached ADU (AADU) Permits in City of Mercer Island
An ADU built inside or attached to the primary residence — basement conversion, garage conversion, or bump-out addition. Lower cost; constrained by the primary footprint. Junior ADUs (JADUs, ≤500 sf) are a subset of this category.
Drops the placement canvas on your parcel for attached adu (aadu) 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
AADU-specific requirements
- Egress on every bedroom — IRC R310 net clear opening ≥5.7 sf; sill ≤44″ from floor.
- Smoke + CO alarms — interconnected throughout primary + AADU per IRC R314/R315.
- 1-hr separation — required between AADU and primary in some cases; otherwise a continuous draft stop.
- Independent kitchen + bath — full plumbing + cooking provision in the AADU.
- Separate address / unit number — required by USPS + 911 dispatch.