Design-Build & Structural Engineering on Mercer Island, WA
ECD works on Mercer Island from our Bellevue office, about 6 miles and 15 minutes away across I-90. Island homeowners most often hire us for foundation and drainage work on sloped lots, seismic upgrades on mid-century houses and remodels that must fit the city's floor area and shoreline rules.
How does Mercer Island review residential projects?
The City of Mercer Island handles building and land use permits, with applications through MyBuildingPermit.com. Interior structural remodels go through building review. Additions, second stories and new structures go through a detailed residential development review that checks gross floor area against lot size, building height, setbacks, lot coverage and tree retention. On smaller island lots, the floor area limit is often what decides how big an addition can be.
Two other reviews are common. Much of the island's perimeter consists of steep slopes and mapped geologic hazard areas, and work there usually needs a geotechnical report. Waterfront lots fall under the city's shoreline master program, which governs work near Lake Washington's edge, including bulkheads, decks and additions toward the water.
Mercer Island houses and what they need
Most island houses were built after the I-90 floating bridge connected the island to Seattle and the Eastside, with a large wave of ramblers, split-levels and mid-century modern houses in the 1950s and 1960s, and larger houses replacing them over time. East Seattle and older parts of the north end have earlier houses. Many island houses sit on sloped lots with daylight basements and views west or east across the lake.
For the engineer, two things stand out. Mid-century houses with walls of glass facing the view often have little shear wall length on that side, so remodels that open more glass or remove walls usually need a lateral upgrade. And the Seattle Fault runs beneath the island, which makes foundation anchorage and bracing on older houses a sensible part of any significant remodel.
Site constraints common on Mercer Island
- Gross floor area limits that cap total house size relative to the lot.
- Steep slopes and geologic hazard areas, especially around the island's perimeter.
- Shoreline review for waterfront lots and work near the lake.
- Tree retention requirements on wooded lots.
- Drainage on hillside lots, where downspout and footing drain discharge must be controlled.
Projects completed on Mercer Island
PROJECT · MERCER ISLANDSettling corner underpinned with helical piers on a sloped lotTen helical piers, a restored footing drain and tightlined downspouts on a daylight-basement house.Mercer Island areas we serve
East Seattle, First Hill, Mercerdale, Mercerwood, North End, Shorewood, South End, The Lakes, Town Center, Westside and Eastside lakefront.
Mercer Island questions
Remodeling on Mercer Island: common questions
How does Mercer Island's floor area limit affect an addition?
Mercer Island limits a house's gross floor area relative to the size of the lot, and counts some basement and garage space toward that total. On many island lots the existing house is already close to the limit, so the allowable addition is smaller than setbacks alone would suggest. We calculate the remaining floor area for your lot before design starts.
Do I need a geotechnical report for work on a Mercer Island slope?
If your lot is in a mapped geologic hazard area and the project involves new foundations, retaining walls or grading, the city typically requires a geotechnical report. Many perimeter lots fall in these areas. Interior remodels usually do not need one. We check the city's hazard mapping for your parcel at the first visit.
Should I seismically upgrade my house during a remodel on the island?
For older wood houses it is usually worth it. The Seattle Fault runs beneath the island, and houses built before modern anchorage and bracing requirements benefit from bolting, cripple wall bracing and hold-downs. When walls are already open for a remodel, adding those upgrades costs much less than a standalone retrofit later.
Can I remodel my waterfront house without shoreline review?
Interior remodels within the existing footprint usually proceed with a building permit alone. Exterior work near the shoreline, such as additions toward the water, decks, bulkheads or docks, falls under the city's shoreline rules and can require a shoreline exemption or permit, which adds time. We identify which applies before design.
Free estimate · Bellevue office
Talk to the engineer before you commit to a plan
A site visit, a scope conversation and a written budget range, at no cost.