Addition permits in City of Mercer Island: fees, timeline & requirements (2026)

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 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

• 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

1. foundation/footing
2. plumbing rough-in
3. electrical rough-in
4. mechanical rough-in
5. framing
6. insulation
7. drywall
8. 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 — Washington baseline, not City of Mercer Island's own schedule

Worked example for a $120,000 home addition project (building permit). Your actual fee scales with project valuation.

Plan-review fee scales with valuation per UBC Table 1-A. State surcharge $6.50 per building permit. Trade permits (E/P/M) separate. 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.