Basement finish permits in Mercer Island: fees, timeline & requirements (2026)
Everything you need to know about pulling a basement finish permit in 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 Mercer Island Community Planning & Development.
Free lookup, no account needed — the office that issues the permit at your address, and a layout of the room, cited to Mercer Island's source.
When does a basement finish need a permit in 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 Mercer Island adopted this rule
The threshold comes from the state code (WAC 51.51), which 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 contractor · Electrician (journey-level 01 or residential 02) · Plumber
Who can do this work in Mercer Island?
Kolmo Construction is a licensed Washington general contractor — KOLMOL*753JS, active through April 10, 2027, bonded and insured — working throughout Mercer Island and King County. Kolmo prepares the drawings and submittals, files the basement finish permit with Mercer Island Community Planning & Development, carries the project through review and inspections, and performs the construction. Call (206) 410-5100 for a 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 estimate in Mercer Island
Worked example for a $40,000 basement finish project (building permit). Your actual fee scales with project valuation.
| Building permit fee | $925 |
| Plan review | $647.50 |
| WA state surcharge | $6.50 |
| Estimated total | $1,579 |
Mercer Island prices a basement finish's building permit on the value of the work: $40.06 up to $500, $118.83 for the first $2,000 plus $23.24 for each $1,000 more up to $25,000, and $671.95 for the first $25,000 plus $16.87 for each $1,000 more up to $50,000. Plan check adds 70% of that fee. Mechanical, electrical and plumbing inspection on the same permit is a combination fee of 31% of the building permit fee (10%, 11% and 10% if permitted separately). The state energy compliance review for a residential alteration is $261 (2026 Fee Schedule). Computed from Mercer Island's adopted fee schedule · source ↗. Land use, fire and engineering review, intake, impact and utility fees, where they apply, are charged separately.
How to apply yourself
- 1. Check who issues the permit. Run an address lookup for your basement finish in Mercer Island — we'll confirm which office issues the permit at that address, and you can lay out the room.
- 2. Assemble submittals: the 6 documents under Required submittals above.
- 3. Submit through the city portal: Mercer Island Community Planning & Development ↗
- 4. Track review. Respond to reviewer comments promptly.
- 5. Pay issuance fees (about $1,579 for a $40,000 project) and pick up the permit. Inspection card travels with the job.
Or have Kolmo do it
Kolmo prepares the drawings and submittals, files with Mercer Island Community Planning & Development, handles reviewer comments, and builds the project.
Licensed WA general contractor KOLMOL*753JS · bonded & insured
Basement finish permits in Mercer Island — FAQ
When does a basement finish need a permit in 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 Mercer Island?
For a $40,000 basement finish project, the estimated Mercer Island permit fee is about $1,579 (building fee $925 + plan review $647.50 + surcharges) from Mercer Island's adopted fee schedule. Land use, fire and engineering review, intake, impact and utility fees, where they apply, are charged separately.