City of Mercer Island building permits: fees, timelines & requirements (2026)
Every fee, threshold, and review window for City of Mercer Island permits — cited to the city's own page and re-verified weekly. Covers ADU, addition, kitchen, bathroom, basement, deck, fence, siding, windows, flooring, roofing, HVAC, electrical, plumbing, and ancillary permits (King County).
14 of 22 project types have authoritative rules on file · 13 typically require a permit · 1 often do not.
Permit department
Apply via MyBuildingPermit.com.
Adopted code cycles
2021 IRC w/ WA amendments · 2021 WSEC
Resources
Construction permits
| Project | Permit? | Threshold | Fee | Review days | Source |
|---|---|---|---|---|---|
| Kitchen remodel | Yes | Cosmetic-only refresh (cabinets, countertops, flooring, paint) without structural/electrical/plumbing changes is generally exempt under 2021 IRC R105.2 (work exempt from permit). Any new circuit, fixture relocation, or structural change requires permits. | Fees set by jurisdiction. WA Building Code Council surcharge: $4.50 per building permit. Trade permits separate. | — | Cite verified 2026-04-29 |
| Bathroom remodel | Yes | Like-for-like fixture replacement in same location with no plumbing rerouting and no new circuits: typically exempt under 2021 IRC R105.2(2)(g). Any relocation, new fixture, new circuit, or shower-pan replacement triggers permits. | Fees set by jurisdiction. Plus separate electrical, plumbing, and mechanical permits. | — | Cite verified 2026-04-29 |
| Basement finish | Yes | 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. | Plan-review fee scales with valuation. State surcharge $4.50. | — | Cite verified 2026-04-29 |
| Deck | Yes | Permit NOT required for sidewalks, decks and driveways constructed under IRC, not more than 30in above grade and not over any basement or story below. Decking replacement on existing decks WITHOUT changing/adding structural members and WITHOUT removing guardrails is also exempt. Per the multi-jurisdiction tip sheet, the 30-inch measurement is taken vertically to the grade below at any point within 36 inches of the outer edge of the deck. Critical-area / flood-hazard exemption rule applies. | Most jurisdictions handle decks as field-inspection or simplified plan review. | — | Cite verified 2026-04-29 |
| Fence | Yes | Fences not over 6 feet (1,829 mm) high are exempt. Maximum fence height per Mercer Island Permit FAQ: 6 feet. | Typically no fee unless permit required. | — | Cite verified 2026-04-29 |
| Siding | Yes | Exempt for non-structural wood and vinyl siding. NOT exempt for veneer, stucco, or exterior finish and insulation systems (EFIS). Exemption does not apply to RCW Chapter 64.55 structures. | Typically minimal fee — like-for-like usually exempt. | — | Cite verified 2026-04-29 |
| Window replacement | Yes | Exempt: in-kind window replacement for IRC structures where no alteration of structural members is required, safety glazing is provided where required, and window fall protection is provided where required. Permit required for: new openings, larger openings, or any structural alteration. | Like-for-like usually no permit fee. Structural changes scale with valuation. | — | Cite verified 2026-04-29 |
| Flooring | Often no | 2021 IRC R105.2(1)(j) exempts floor finishes (carpet, vinyl, hardwood, tile). Permit required if subfloor structure is modified, joists altered, or radiant heat installed. | No permit fee for surface flooring. | — | Cite verified 2026-04-29 |
| ADU / DADU | Yes | WA HB 1337 (effective 2024) requires cities ≥6,000 pop. to allow 2 ADUs per lot in residential zones, max 1,000 sq ft, no owner-occupancy requirement, no parking requirement near transit. Local size + design caps apply. | Full Construction Permit (Addition/Alteration) fees. Sewer capacity charges may apply (King County: ~$8k-$10k). | 30–120 days | Cite verified 2026-04-29 |
| Roofing | Yes | Permit required when: sheathing is replaced OR the type of roofing material is changed. Exempt: in-kind re-roofing of one- and two-family dwellings provided the roof sheathing is not removed or replaced. | Flat fee or valuation-based. Typically $150-$400 for residential. | — | Cite verified 2026-04-29 |
| HVAC | Yes | WAC 51-52 (2021 IMC) and WAC 51-56 (2021 UPC) require mechanical permit for any equipment install, replacement, or relocation. WSEC R403 requires sizing per Manual J/S/D for new equipment. | Mechanical permit + electrical permit (if new circuit) + plumbing permit (if condensate to drain). | — | Cite verified 2026-04-29 |
| Electrical | Yes | WAC 296-46B (WA-adopted 2020 NEC) requires electrical permit for ALL electrical work except low-voltage (<50V) and ordinary repairs. WA L&I issues electrical permits in most jurisdictions; some cities have local issuance. | L&I permit fees by circuit count + service size. Standard residential ~$60-$200. | — | Cite verified 2026-04-29 |
| Plumbing | Yes | WAC 51-56 (2021 UPC) requires plumbing permit for installation, replacement, or relocation of any plumbing fixture, water heater, or piping. Like-for-like faucet replacement is exempt. | Plumbing permit fees by fixture count. Standard residential ~$80-$250. | — | Cite verified 2026-04-29 |
| Addition | Yes | Mercer Island distinguishes additions <500sqft (no Intake Screening required) from additions ≥500sqft (Substantial Residential Project, may require Intake Screening prior to review). | — | — | Cite verified 2026-04-29 |
How this page is built
Each row is the most-recently-verified authoritative rule on file for City of Mercer Island and the listed project type. “Pending verification” means we have City of Mercer Island in our database but haven't yet personally checked the city page for that project — those rows fall back to King-County or Washington-state baselines until verified.
See the data-quality leaderboard for how we keep these current, or run an address-specific lookup on the main permits page.