City of Mercer Island, WA

Common application types

• Building
• Demolition
• Grading
• Stormwater
• Site Development
• Tree
• Electrical
• Mechanical
• Plumbing
• Water Service

ADUs and middle housing

Mercer Island adopted middle housing and ADU regulations (Ordinance No. 25C-02) effective June 30, 2025, which the City described as interim, with permanent rules to follow further outreach beginning in 2026. Lots allow a single-family home plus at least 2 middle housing units, or up to 4 within a quarter mile of the light rail station or with at least one affordable unit. Up to two attached or detached ADUs are allowed per lot under MICC 19.02.030, sized 220 to 1,000 sq ft excluding garage area, and the City removed the ADU owner occupancy requirement. ADUs are reviewed as a Type II land use application. Ordinance 25C-06 allows unit lot subdivision of a parent lot into up to four unit lots for separate sale.

Land use review

Unit lot subdivision uses the City's short plat process under Ordinance 25C-06. Ordinance 25C-14 dissolved the Design Commission. An omnibus code package to make interim housing and permit-timeline regulations permanent, including SB 5290 permit review timelines in MICC 19.15, received a Planning Commission recommendation in October 2025 and went to City Council beginning November 2025.

Corrections and review

The City publishes target review timelines as estimates. Residential building permits target 8 to 12 weeks for first review, 3 weeks for second review and 2 weeks for later reviews; commercial building permits target 12 weeks for first review. Land use decisions target 4 weeks for completeness review and 8 to 12 weeks for first review. Residential building permits that require public notice under MICC 19.15.030 have a 30-day comment period that must end before review is completed.

Tips from SASHKOV

• Building permit applications expire if no permit is issued within 18 months.
• Issued building permits expire if work is not completed within two years from date of issue.
• Total accessory building gross floor area, including ADUs, may not exceed 25% of the gross floor area allowed on the lot under MICC 19.02.040.
• Pay intake fees within 7 days of the email confirming the application is complete.

Where to apply

Permitting authority: Community Planning & Development (CPD). Online submittals: Mercer Island Permit Submittal Portal (MIePlan electronic plan review).