Addition permits in City of Mercer Island: fees, timeline & requirements (2026)
Everything you need to know about pulling a home addition 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..
When does a home addition need a permit in City of Mercer Island?
Mercer Island distinguishes additions <500sqft (no Intake Screening required) from additions ≥500sqft (Substantial Residential Project, may require Intake Screening prior to review).
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
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 City of Mercer Island
Worked example for a $120,000 home addition project (building permit). Your actual fee scales with project valuation.
| Building permit fee | $1,105.75 |
| Plan review | $718.74 |
| WA state surcharge | $4.5 |
| Estimated total | $1,828.99 |
Plan-review fee scales with valuation per UBC Table 1-A. State surcharge $4.50 per building permit. Trade permits (E/P/M) separate. Computed from the City of Mercer Island fee bracket schedule · source ↗. Impact/utility fees may apply separately.
How to apply
- 1. Confirm your parcel's zoning & overlays. Run an address lookup for your City of Mercer Island home addition — we'll pull your specific lot polygon, zoning, setbacks, and any shoreline/ECA/historic overlays.
- 2. Assemble submittals (11).
- 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 (Plan-review fee scales with valuation per UBC Table 1-A. State surcharge $4.50 per building permit. Trade permits (E/P/M) separate.) and pick up the permit. Inspection card travels with the job.
Addition permits in City of Mercer Island — FAQ
When does a home addition need a permit in City of 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 City of Mercer Island?
For a $120,000 home addition project, the estimated City of Mercer Island permit fee is about $1,828.99 — building fee $1,105.75 + plan review $718.74 + surcharges. Fees scale with project valuation per the city schedule.
How long does City of Mercer Island take to review a home addition permit?
Typical review window: 30–120 days.