Basement finish permits in City of Mercer Island: fees, timeline & requirements (2026)
Everything you need to know about pulling a basement finish 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..
Free lookup, no account needed — your lot polygon, zoning, setbacks and overlays, cited to City of Mercer Island's source.
When does a basement finish need a permit in City of Mercer Island?
2021 IRC R310: every sleeping room below grade requires emergency escape and rescue opening (egress window). Min sill height 44in, min net clear opening 5.7 sq ft (5.0 grade-floor), min width 20in, height 24in. Ceiling height min 7ft (R305). Entire newly conditioned area must comply with WSEC R402.
How City of Mercer Island adopted this rule
The threshold comes from the state code (WAC 51.51), which City of Mercer Island adopts by reference in the provision quoted below.
“The 2021 Edition of the International Building Code (IBC), including the adoption of ICC/ANSI A117.1-2017, Requirements for Accessible and Useable Buildings and Facilities, as adopted and amended by the State Building Code Council in WAC Chapter 51-50, as published by the International Code Council, excluding Chapter 1, Administration, is adopted by reference, together with the following amendments and additions. The Construction Administrative Code, as set forth in chapter 17.14 MICC, shall be used in place of IBC Chapter 1, Administration.”Source ↗verified 2026-04-29
Required submittals
- floor plan w/ egress windows + ceiling heights
- electrical layout
- plumbing diagram
- mechanical / HVAC plan
- WSEC compliance form (newly conditioned space)
- smoke/CO alarm locations
Inspection sequence
- rough-in (electrical/plumbing/mechanical)
- framing
- insulation
- drywall
- final
Contractor specialties needed
general, electrical-01-or-02, plumbing
Who can do this work in City of Mercer Island?
Kolmo Construction is a licensed Washington general contractor — KOLMOL*753JS, active through April 10, 2027, bonded and insured — working throughout City of Mercer Island and King County. Kolmo prepares the drawings and submittals, files the basement finish permit with Apply via MyBuildingPermit.com., carries the project through review and inspections, and performs the construction. Call (206) 410-5100 for a City of Mercer Island basement finish quote.
Notes & caveats
Per Mercer Island Residential Building Permits page (separate "Interior Remodel - Structural" listing): scope is "limited to the interior affecting the structure of a building. I.e. Beams, posts, or lateral resistance." Basement conversion to habitable space typically involves egress windows, framing, and electrical/plumbing — falls under Structural Interior Remodel.
Basement finish permit fee — Washington baseline, not City of Mercer Island's own schedule
Worked example for a $40,000 basement finish project (building permit). Your actual fee scales with project valuation.
| Building permit fee | $542.75 |
| Plan review | $352.79 |
| WA state surcharge | $6.5 |
| Estimated total | $902.04 |
Plan-review fee scales with valuation. State surcharge $6.50. We have not read City of Mercer Island's adopted fee schedule, so this is not City of Mercer Island's fee: it walks the standard valuation table most Washington cities adopt by reference, as a rough guide. The real numbers are in City of Mercer Island's fee schedule ↗. Impact/utility fees may apply separately.
How to apply yourself
- 1. Confirm your parcel's zoning & overlays. Run an address lookup for your City of Mercer Island basement finish — we'll pull your specific lot polygon, zoning, setbacks, and any shoreline/ECA/historic overlays.
- 2. Assemble submittals (6).
- 3. Submit through the city portal: Apply via MyBuildingPermit.com. ↗
- 4. Track review. Respond to reviewer comments promptly.
- 5. Pay issuance fees (Plan-review fee scales with valuation. State surcharge $6.50.) and pick up the permit. Inspection card travels with the job.
Or have Kolmo do it
We pull basement finish permits in City of Mercer Island every month. We prepare the drawings and submittals, file with Apply via MyBuildingPermit.com., handle reviewer comments, and build the project.
Licensed WA general contractor KOLMOL*753JS · bonded & insured
Basement finish permits in City of Mercer Island — FAQ
When does a basement finish need a permit in City of Mercer Island?
2021 IRC R310: every sleeping room below grade requires emergency escape and rescue opening (egress window). Min sill height 44in, min net clear opening 5.7 sq ft (5.0 grade-floor), min width 20in, height 24in. Ceiling height min 7ft (R305). Entire newly conditioned area must comply with WSEC R402.
How much does a basement finish permit cost in City of Mercer Island?
For a $40,000 basement finish project, the estimated City of Mercer Island permit fee is about $902.04 — building fee $542.75 + plan review $352.79 + surcharges. Fees scale with project valuation per the city schedule.