Addition permits in Mercer Island: fees, timeline & requirements (2026)
Everything you need to know about pulling a home addition 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 — your lot polygon, zoning, setbacks and overlays, cited to Mercer Island's source.
When does a home addition need a permit in Mercer Island?
Mercer Island distinguishes additions <500sqft (no Intake Screening required) from additions ≥500sqft (Substantial Residential Project, may require Intake Screening prior to review).
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
- Site plan with north arrow, property lines, setbacks, existing + proposed structures, lot coverage calculation
- Floor plan existing and proposed (dimensioned, 1/4" = 1' min)
- Elevations all affected sides (existing + proposed, height-to-grade)
- Foundation plan + footing details (if footprint addition)
- Framing plan (floor + roof + lateral)
- Wall section with insulation values per WSEC
- Structural calculations (when ≥500 sqft addition or out of IRC prescriptive envelope)
- WSEC compliance form (Energy Code Compliance Certificate)
- Manual J HVAC sizing calculation for new conditioned area
- Window/door schedule with NFRC U-factor and SHGC values
- Soils report if footing on slope or expansive soils
Inspection sequence
- Foundation/footing
- Plumbing rough-in
- Electrical rough-in
- Mechanical rough-in
- Framing
- Insulation
- Drywall
- Final
Contractor specialties needed
General contractor
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 home addition 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 home addition quote.
Notes & caveats
Per Mercer Island Residential Building Permits page, two addition pathways are listed separately: under "Addition - Less than 500 Square Feet" the page states "If your project meets the criteria, an Intake Screening may not be required." Under "Addition - 500 Square Feet or More" the page states "Considered a Substantial Residential Project: May require an Intake Screening prior to review." Submittal pathways and review timelines differ between the two.
Addition permit fee estimate in Mercer Island
Worked example for a $120,000 home addition project (building permit). Your actual fee scales with project valuation.
| Building permit fee | $1,891.96 |
| Plan review | $1,324.37 |
| WA state surcharge | $6.50 |
| Estimated total | $3,222.83 |
Mercer Island prices an addition's building permit on the value of the work: $1,091.17 for the first $50,000 plus $11.67 for each $1,000 more up to $100,000, and $1,704.76 for the first $100,000 plus $9.36 for each $1,000 more up to $500,000. 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). Intake screening is $903, the state energy compliance review for a residential alteration $261, and fire review of a remodel $182 (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. Confirm your parcel's zoning & overlays. Run an address lookup for your home addition in Mercer Island — we'll pull your specific lot polygon, zoning, setbacks, and any shoreline, critical-area or historic overlays.
- 2. Assemble submittals: the 11 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 $3,223 for a $120,000 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
Addition permits in Mercer Island — FAQ
When does a home addition need a permit in Mercer Island?
Mercer Island distinguishes additions <500sqft (no Intake Screening required) from additions ≥500sqft (Substantial Residential Project, may require Intake Screening prior to review).
How much does a home addition permit cost in Mercer Island?
For a $120,000 home addition project, the estimated Mercer Island permit fee is about $3,222.83 (building fee $1,891.96 + plan review $1,324.37 + 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.