Mercer Island Home Remodeling

35.1 percent of Mercer Island homes were built in the 1950s and 1960s

The Census Bureau's 2020 to 2024 estimates put 10,609 homes on the island. The 1960s produced 2,506 of them and the 1950s another 1,215, and the median home was built in 1973. A postwar house was built long before today's energy code, so opening a wall can mean adding insulation, replacing old wiring and upgrading windows on top of the finishes you picked. Leave room in the budget for what the walls reveal.

Source: Census ACS 2020-2024, Year Structure Built, Mercer Island

At least two strands of the Seattle Fault cross Mercer Island

The city's emergency management office says at least two strands of the Seattle Fault run across the island. Mercer Island maps landslide, steep slope, seismic and erosion hazard areas, and building on those lots takes a geotechnical report stamped by a Washington licensed geotechnical, geological or structural engineer, including a statement of risk. On affected lots, land clearing, grading, filling and foundation work are not allowed between October 1 and April 1. A waiver is possible only with compelling justification backed by a geotechnical evaluation.

Source: City of Mercer Island, Geological Hazards

Mercer Island requires a permit to cut any tree 10 inches or larger

Under chapter 19.10 of the city code, a tree whose trunk is under 10 inches across at 4.5 feet can be cut without a permit unless they are exceptional trees or stand in a critical area. Larger trees need one, and so does any tree cut for construction. Replacements scale with size: two new trees for a 10 to 24 inch tree, three for 24 to 36 inches, and six for anything bigger or exceptional, kept alive for five years. Development must keep at least 30 percent of large trees and every exceptional tree 24 inches and up.

Source: City of Mercer Island, Tree Frequently Asked Questions (MICC 19.10)

How much are permit fees for a remodel on Mercer Island?

Under the 2026 fee schedule, a $150,000 remodel pays a building permit fee of $2,172.76. Plan check adds 70 percent of that, and a combined mechanical, electrical and plumbing inspection fee adds 31 percent, for $4,367.25 in total. Land use, tree or critical area reviews are charged separately when the project needs them.

When can I do excavation or foundation work on Mercer Island?

On lots in mapped geologic hazard areas, land clearing, grading, filling and foundation work are not allowed between October 1 and April 1. Tree cutting in those areas follows the same calendar. The city can grant a waiver, but only with compelling justification supported by a geotechnical evaluation, so schedule digging for spring and summer.

Which trees need a cutting permit on Mercer Island?

Any tree measuring 10 inches or more across at 4.5 feet needs one, any exceptional tree and any tree in a critical area. Smaller ordinary trees need no permit. Removed trees are replaced at two to six new trees each, depending on size, and the replacements must be kept healthy for five years.

Is Mercer Island on the Seattle Fault?

Yes. The city's emergency management office says at least two strands of the Seattle Fault cross the island. Building in a mapped seismic, landslide, steep slope or erosion hazard area needs a geotechnical report stamped by a Washington licensed engineer with a statement of risk. The city's GIS map shows whether a lot is in one.

How do I get an ADU approved on Mercer Island?

An ADU first goes through a Type II land use review, a decision made by the code official without a public hearing. The January 2026 checklist asks for an application form, a project narrative, a code compliance matrix against MICC 19.02.030, a title report under 30 days old and a plan set. The building permit follows.