An island with its own rules, and a lot of 1965 houses
Much of Mercer Island's housing was built between the late 1950s and the 1970s: solid, well-sited homes with layouts that no longer match how anyone lives, single-pane windows, minimal insulation and kitchens closed off from everything. They are excellent renovation candidates. The bones are typically good and the lots are worth the investment.
What makes Mercer Island different from any other city we serve is the regulatory texture. The island's tree code is among the more protective in the region, shoreline properties fall under the Shoreline Master Program, and everything and everyone — crews, materials, dumpsters — arrives across one of two bridges. That last one is a logistics fact, not a legal one, but it belongs in an honest schedule.
Neighborhoods we work in
First Hill · East Seattle · Mercerwood · Island Crest · North End · South End · Shoreline properties
Permits and inspections on Mercer Island
The City of Mercer Island reviews and permits residential work. Two local factors come up repeatedly: tree regulations, which govern removal of and construction near significant trees and typically require arborist documentation, and shoreline rules for waterfront parcels, where work within the shoreline jurisdiction is reviewed under the city's Shoreline Master Program and can add substantial time to the front of a project.
Steep lots descending to the water can also require geotechnical review. None of this makes projects impossible — it makes early, accurate planning worth far more here than in a flat inland city. We confirm current requirements with the city for each project rather than relying on last year's rules.
What we build in Mercer Island
- Full renovations of 1960s and 70s island homes, including insulation and systems
- Kitchen remodels that finally open the plan to the view
- Primary suite additions and second-story work within height limits
- Bathroom rebuilds with proper waterproofing and modern ventilation
- Decks, stairs and outdoor living space on sloped, treed lots
- Window and envelope upgrades that make a mid-century house comfortable
Mercer Island at a glance
- Based in Kirkland — Mercer Island is a core service area
- We pull the permits and meet the inspectors
- Written, itemized estimates — no verbal numbers
- Bonded and insured in Washington
- Free walkthrough, written estimate
What Mercer Island lets you build in the back yard, in 2026
Washington rewrote the accessory dwelling unit rules statewide, and Mercer Island rewrote its code to match. Most of what homeowners here believe about ADUs is two or three years out of date.
| What people ask | Mercer Island, 2026 |
|---|---|
| How many you can build | Up to two, attached or detached |
| Biggest detached unit | 1,000 sq ft, minimum 220 sq ft |
| Height | At least 24 ft |
| Parking | None within half a mile of light rail. Otherwise one space per unit on a lot of 6,000 sq ft or less, two per unit on a larger lot |
| Can you sell it separately | Yes — an ADU on a unit lot may be sold individually, and a condominium built as an ADU may be conveyed separately |
| Owner occupancy | Not required. Washington abolished it statewide under RCW 36.70A.681 — you do not have to live on the lot to have an ADU, and you may rent both units |
| Who issues the permit | City of Mercer Island Community Planning and Development — an ADU is a Type II land use review |
Worth knowing in Mercer Island
Mercer Island's ADU rules are interim, adopted as Ordinance 25C-02 in March 2025 and effective from June 2025. The city has said it intends to adopt permanent regulations after more public engagement, with that work starting in 2026. Anything you design now should be checked against the rules in force on the day you submit.
Ready-made plans
Mercer Island has no pre-approved plan programme.
Source: MICC 19.02.030(B), interim regulations under Ordinance 25C-02, plus RCW 36.70A.681. Checked 4 September 2026. Codes in this region are being rewritten faster than any web page keeps up with — we verify your parcel against the code in force on the day we submit, not against a summary.
Four ways to add a unit — a new detached building, the garage, the basement or a suite inside the house — with a calculator that takes your own numbers: see all four and what each returns →
What it costs: a new detached unit on the Eastside runs roughly $325,000 to $550,000 all in, and advertised "starting at" prices leave out $120,000 to $180,000 of site, utility and permit work. Here is the whole number, line by line →
Building in Mercer Island: what works in your favor
- Mercer Island permits are filed through the regional MyBuildingPermit portal — one system we work in every week.
- Large established lots make DADUs and major additions particularly viable — and particularly valuable — on the Island.
Statewide, Washington law (HB 1337) is on your side in Mercer Island: up to two ADUs are allowed on most residential lots, a detached ADU may be up to 24 ft tall with side and rear setbacks of no more than 5 ft, units up to 1,000 sq ft must be permitted, there is no owner-occupancy requirement, and no parking can be required near major transit.
Rules change and every lot is different — we verify your exact address with the city before any design work. This page is information, not legal advice.
Mercer Island questions we get asked
Will the tree code stop my project?
We're on the waterfront. Is that a different process?
Are you set up to work on the island?
Nearby areas we serve
Building in Mercer Island? Let's talk.
Tell us about your project — we'll listen, come out, and give you an honest number.
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Tell us about your project — we'll come out, listen, and give you an honest number.
Call or text(425) 866-3940Email: office@nwhomesremodel.com
Hours: Mon–Fri 8:00 AM – 6:00 PM · Sat by appointment
Based in Kirkland, WA — serving the Eastside and Greater Seattle