MNBE designs, permits, and builds ADUs, DADUs, and custom homes across Mercer Island, one licensed team from first site visit to final inspection.
One licensed, bonded team for the whole job, from design and Mercer Island Community Planning & Development permitting to fixed price construction.
Where a First Hill or Mercerwood lot's land value justifies it, we replace an aging home with a ground up custom home, with feasibility run honestly first.
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Detached, attached, and basement ADUs, up to two per lot and 1,000 sq ft each under Mercer Island's 2025 interim middle housing code.
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Drawings built to clear Mercer Island Community Planning & Development the first time, steep slope view lots and critical areas included.
Explore ServiceWhy MNBE in Mercer Island
We design ADU submittals to land clean at Mercer Island's Community Planning and Development, all routed through the MyBuildingPermit portal, anticipating the corrections their reviewers send back before we file. CPD targets roughly two weeks for first review of non major residential permits, and a clean first submittal keeps you in that lane instead of a correction loop. Every submittal and every response is handled by us, never a third party expediter.
Mercer Island is a glacial island of hills, ravines, and waterfront lots, and the streets with the best lake, Cascade, and skyline views in First Hill, Mercerwood, and Shorewood are often the most regulated. We coordinate the geotechnical report early, design special foundations and drainage to the slope, and run geologic hazard, critical area, and any shoreline review in parallel with the building permit. We also plan around the Oct 1 to Apr 1 grading moratorium on mapped erosion and slide hazard lots so it does not stall the project.
Our attached ADU in nearby Bellevue, just across Lake Washington, full framing, roofing, kitchen, and HVAC, finished in 2024 and passed every City of Bellevue inspection. We build the same way on Mercer Island. See the full project, and the homeowner's verified five star review, on the project page.
What We Build in Mercer Island
A freestanding backyard cottage with its own entrance and address, the strongest rental performer. Mercer Island's interim rules allow detached ADUs up to 1,000 square feet, with no published stock plan program yet, so a clean custom set is the path. The larger estate lots in Shorewood and along Island Crest take one comfortably, though the tree code and 30 percent Large Tree retention rule shape where it can sit.
Connected to the main house by a shared wall or addition. Mercer Island's interim code allows up to two ADUs per lot, removed the prior owner occupancy requirement, and waives added parking within a half mile of the I-90 light rail station, so both units can be rented. Often the faster permit on a tight Town Center lot.
The post WWII mid century ramblers and contemporary homes of Mercerwood, East Seattle, and the North End sit on the daylight and walkout basements that hillside siting makes common here, and they convert beautifully into legal dwellings with a separate entrance, full kitchen, and code compliant egress. These conversions count toward the two ADU allowance.
On many Mercer Island lots, especially the waterfront streets of Shorewood and the view blocks of First Hill, land value justifies replacing an aging home outright. We run that feasibility honestly, sometimes the right answer is a ground up custom home, not a remodel, and build it under the same license and fixed price contract.
FAQ
Answering the questions Mercer Island homeowners ask us most about building an ADU.
Yes. Under Mercer Island's interim regulations and Washington's HB 1337, most residential lots allow up to two ADUs in any mix of attached, detached, or conversion, with no owner occupancy requirement except when an ADU is used as a short term rental. These interim rules were adopted in 2025, with permanent regulations expected in 2026, so we verify current numbers against MICC 19.02.030.
Mercer Island publishes target times of about two weeks for first review of non major residential permits and about two weeks per resubmittal, generally faster than Seattle SDCI thanks to a smaller queue. That advantage narrows when geologic hazard, critical area, or shoreline review applies, or a public notice comment period is required. Clean lots run fast; steep or waterfront lots don't.
Cost is site specific on Mercer Island, so we don't quote a generic per square foot number. The real drivers are slope and geotechnical conditions, special foundations like pin piles or stepped daylight footings, clay soils and shallow groundwater that often rule out simple stormwater infiltration, tree retention, and shoreline or geologic hazard jurisdiction. You get a fixed price after a free site visit.
Thank you for considering MNBE for your Mercer Island ADU or custom home. We respond within one business day, Monday through Saturday.
We design and build across Mercer Island, from the sub 6,000 square foot legacy lots near Town Center and the walkable station core to the steep view streets of First Hill and Mercerwood and the waterfront blocks of Shorewood and the North End above Lake Washington. On a glacial island of hills, ravines, clay soils, and widespread critical areas, we plan foundations, drainage, tree retention, and the seasonal grading moratorium into the design from day one.