How long does City of Mercer Island take to review a ADU / DADU permit?

Typical review window: 30–120 days.

Which cities is the Kolmo permit lookup verified for?

Ten cities: Seattle, Bellevue, Kirkland, Redmond, Sammamish, Issaquah, Renton, Kent, Federal Way, and Mercer Island. For these, every fee, threshold, and timeline is sourced from the city directly with the exact quote and URL, and re-verified weekly. Outside these ten cities address lookup still works and we render the parcel and Washington-state baseline code, but per-city specifics are flagged as unverified.

Does the lookup cover unincorporated King, Pierce, or Snohomish County?

Address lookup works and parcels resolve to the county building department, but unincorporated coverage is currently baseline-only — Washington-state IRC, IBC, and WSEC plus state critical-area rules. Per-county fee and code deviations have not been verified to the same standard as the ten verified cities.

Will this replace calling the permit center?

For most homeowner projects — decks, kitchens, bathrooms, ADUs, fences, siding, windows, flooring, roofing, HVAC, electrical, plumbing — the published answer matches what the permit center will say, and the homeowner can submit with a pre-filled application packet. For variances, conditional uses, or unusual scope, calling the city planner is still recommended.

Where does the address go after submission?

The address is geocoded via Google Maps to identify the correct county, then queried against public county GIS endpoints. Parcel data is cached for 30 days. The address is not contacted, sold, or shared with third parties.

Who built this — and why?

Built by Kolmo Construction, a licensed Seattle-area general contractor. The team works with permit code daily and built this to fix the public web's poor coverage of homeowner permit questions. The lookup is free; if a homeowner wants a contractor for the project, Kolmo is one option.

ADU / DADU permits in City of Mercer Island: fees, timeline & requirements (2026)

Everything you need to know about pulling a ADU / DADU permit in City of Mercer Island, Washington — when one is required, what it costs, how long review takes, what documents are required, and which inspections you can expect. Cited to Apply via MyBuildingPermit.com..

Permit required?

Yes — see threshold below

Permit fee

Full Construction Permit (Addition/Alteration) fees. Sewer capacity charges may apply (King County: ~$8k-$10k).

Review timeline

30–120 days

When does a ADU / DADU need a permit in City of Mercer Island?

WA HB 1337 (effective 2024) requires cities ≥6,000 pop. to allow 2 ADUs per lot in residential zones, max 1,000 sq ft, no owner-occupancy requirement, no parking requirement near transit. Local size + design caps apply.

Required submittals

full architectural plans
structural calcs
WSEC compliance form
site plan w/ setbacks
sewer/water capacity certification

Inspection sequence

footing
foundation
framing
rough-in (electrical/plumbing/mechanical)
insulation
drywall
final

Contractor specialties needed

general

Notes & caveats

From Mercer Island Permit FAQ: "DO I NEED TO APPLY FOR AN ACCESSORY DWELLING UNIT PERMIT? An ADU permit will be required if you are wanting to create a habitable dwelling unit added to, created within, or detached from a single-family dwelling that provides basic requirements for living, sleeping, eating, cooking and sanitation. (* Commonly referred to as DADU / ADU )." Specific Mercer Island setback/size/parking requirements live in MICC Title 19 (Land Use) — not directly cited in cached pages, so this rule documents the permit-required threshold but defers numeric thresholds to Mercer Island Land Use Code review.

How to apply

1. Confirm your parcel's zoning & overlays. Run an address lookup on the main permits page — we'll pull your specific lot polygon, zoning, setbacks, and any shoreline/ECA/historic overlays.
2. Assemble submittals (5).
3. Submit through the city portal: Apply via MyBuildingPermit.com. ↗
4. Track review (typical: 30–120 days). Respond to reviewer comments promptly.
5. Pay issuance fees (Full Construction Permit (Addition/Alteration) fees. Sewer capacity charges may apply (King County: ~$8k-$10k).) and pick up the permit. Inspection card travels with the job.

Compare this against other cities

Side-by-side ADU / DADU requirements across all 10 verified Seattle-area cities.

All City of Mercer Island permits

Every project type — ADU, addition, kitchen, bath, deck, fence, and more — for City of Mercer Island.

City of Mercer Island resourcesPermit portal ↗Fee schedule ↗