Mercer Island building permits, zoning and tree code: what you can build in 2026

Short answer: on Mercer Island, what you can build is decided by five things, in roughly this order. Your zone sets the floor area. Lot width sets the side yards. Slope sets lot coverage. The trees on the lot set where the house can go. And if you are on the water, the shoreline code sets how close you can get to it. The permit itself comes last, and it moves fastest when all five were settled before anyone drew a plan.

For how I work on the island, see the Mercer Island architect page. Every number below comes from the Mercer Island City Code (MICC) or the city’s own permit pages as of September 2026. Codes change, so confirm with the city before you rely on any figure.

Which residential zone your lot is in

Single-family Mercer Island is zoned R-8.4, R-9.6, R-12 or R-15, and the residential development standards in MICC 19.02 apply to all four. The number is the minimum lot size in thousands of square feet. You can look up your zone and lot size on the city’s online map portal (search the address, then open the parcel’s Property Report and Property Detail).

Zone	Minimum lot	Minimum width	Floor area cap
R-8.4	8,400 sq ft	60 ft	5,000 sq ft or 40% of the lot, whichever is less
R-9.6	9,600 sq ft	75 ft	8,000 sq ft or 40% of the lot, whichever is less
R-12	12,000 sq ft	75 ft	10,000 sq ft or 40% of the lot, whichever is less
R-15	15,000 sq ft	90 ft	12,000 sq ft or 40% of the lot, whichever is less

Source: MICC 19.02.020(A) and (D). Minimum depth is 80 feet in every zone. Lots created before September 28, 1960 are not held to the minimum lot area, though other conditions apply before they can be built on.

The 40 percent figure binds first until the lot gets large. The fixed cap only takes over above 12,500 square feet of lot in R-8.4, 20,000 in R-9.6, 25,000 in R-12 and 30,000 in R-15 (each cap divided by 0.40). So a 20,000 square foot R-15 lot allows 8,000 square feet of gross floor area (20,000 x 0.40), well under the 12,000 square foot cap. Lots of 7,500 square feet or less get a small allowance, the lesser of 3,000 square feet or 45 percent.

One more rule for big lots: on a lot large enough to subdivide (at least 16,800, 19,200, 24,000 or 30,000 square feet by zone), a new house must leave room for a future subdivision, follow one, or come with a recorded five year no-subdivision notice (MICC 19.02.020(I)).

Setbacks, and the side yard rule tied to lot width

The front yard is 20 feet and the rear yard 25 feet. Side yards are set by lot width and wall height rather than a flat number.

• Total side yards. On a lot 90 feet wide or less, the two side yards must add up to at least 15 feet. Wider than 90 feet, they must add up to 17 percent of the lot width. A 100 foot wide lot needs 17 feet in total.
• Minimum on either side. Each side yard must be at least 5 feet or 33 percent of the total, whichever is greater. On that 100 foot lot, 33 percent of 17 feet is about 5.6 feet, so neither side can drop below that.
• The tall wall rule. On lots of 6,000 square feet or more, a side wall taller than 15 feet (or a gable end taller than 18 feet) pushes that side yard, where it abuts a neighboring lot, to at least 7.5 feet. A side wall taller than 25 feet pushes it to 10 feet. Height is measured from existing or finished grade, whichever is lower.

That last rule is the one that changes designs. A two-story side elevation almost always clears 15 feet, so on a narrow lot the real side yard is often 7.5 feet, not 5.

On a waterfront lot, the code lets the front yard be measured from the property line opposite the water, whatever side the driveway is on.

How big and how tall: floor area, height and lot coverage

Gross floor area is measured to the outside faces of the walls and includes garages, detached accessory buildings over 120 square feet, the part of a basement that projects above grade, staircases and covered upper-level decks. Tall rooms cost you: floor area under a ceiling of 12 to 16 feet counts at 150 percent, and above 16 feet at 200 percent. A double-height living room can quietly use up a large share of the allowance.

Height is limited to 30 feet above the average building elevation, which is the weighted average of grade at the midpoint of each wall segment. On a sloping lot there is a second test: the downhill facade may not exceed 30 feet from grade to the top of the wall.

Lot coverage (house, driveways and accessory buildings) depends on slope:

Lot slope	Maximum lot coverage	Required landscaping
Under 15%	40%	60%
15% to under 30%	35%	65%
30% to 50%	30%	70%
Over 50%	20%	80%

Source: MICC 19.02.020(F). Up to 9 percent of the lot may be hardscape such as walkways and decks, within the landscaping area.

The tree code (MICC 19.10)

• Tree permit. Removing any tree of 10 inches or more in diameter generally needs a permit. Smaller trees are exempt unless they are exceptional trees, earlier replacement trees, or in a critical area or buffer.
• Retention on a new house. A new house, or an addition of more than 500 square feet, on a lot of 6,000 square feet or more must retain at least 30 percent of the trees of 10 inches or more, counted over a rolling five years.
• Exceptional trees of 24 inches or more (exceptional status is set by species and size in the code’s table) must be kept unless a narrow exception applies, such as an unavoidable hazard, or keeping the tree would hold the buildable floor area under 85 percent of what the zone allows.
• Replacement. Removed trees are replaced at 1 to 6 trees each, rising with diameter, planted between October 1 and April 1 and maintained for five years.
• Clearing ahead of time backfires. A lot that has lost more than 70 percent of its large trees in the previous five years cannot get development approval until it complies.

Get an arborist’s tree survey before design starts; the retained trees become fixed points the house is designed around.

Waterfront lots and the Shoreline Master Program

Waterfront is the core of my work. The shoreline rules in MICC 19.13 apply to “shorelands,” which the code defines as Lake Washington and the land within 200 feet of its ordinary high water mark (OHWM). They sit on top of every zoning rule above.

• Setback. All structures, including fences over 48 inches, sit at least 25 feet from the OHWM, in addition to the zoning yards.
• Coverage near the water. Hardscape and lot coverage are limited to 10 percent of the area within 25 feet of the OHWM and 30 percent between 25 and 50 feet.
• Height. The shoreline cap is 35 feet, but it defers to the zoning limit, so for a house the 30 foot rule governs.
• Native planting. Adding 500 square feet or more of floor area, coverage or hardscape (counting the previous five years) requires native vegetation over 50 percent of a 20 foot strip along the water. At 1,000 square feet or more it becomes 75 percent. On a rebuild, settle early with the city how much of the new area counts as “additional.”
• Existing houses close to the water. A legally built structure within 25 feet of the OHWM may be maintained, remodeled and even completely replaced, as long as the nonconformity is not increased. Expansions must meet current rules. That allowance covers the shoreline standards; zoning yards still follow the 40 percent wall rule below.

Docks and bulkheads are regulated separately. One residential moorage is allowed per waterfront lot, set back 10 feet from the lateral line (your side property line extended into the lake), generally no more than 100 feet out, and generally no wider than 4 feet within the first 30 feet of the OHWM. A covered moorage is limited to 600 square feet and 16 feet high unless extra criteria or permits are met, with open sides and translucent roofing. Replacing a bulkhead requires a demonstrated need, and a new hard bulkhead for an existing house needs a geotechnical analysis showing real erosion risk from waves or currents. Several dock scenarios also need U.S. Army Corps of Engineers and Washington Department of Fish and Wildlife approval.

A single-family house is listed in the shoreline use table as permitted through a shoreline exemption rather than a substantial development permit. It still has to meet every shoreline standard. How I approach the design side is on the Lake Washington lakefront design page, and the Medina shoreline guide is a useful comparison if you are weighing lots in both cities.

Steep slopes and other critical areas

The city notes it is very common for an island project to touch a mapped geologic hazard area. The code defines a steep slope as 40 percent or more, measured over any 30 foot horizontal run. Steep slopes carry a buffer at the top and the toe equal to the height of the slope, up to 75 feet. Shallow landslide hazard areas carry a 25 foot buffer and deep-seated ones 75 feet.

Building in these areas is allowed, but it needs a critical area study by a geotechnical professional showing the site will be safe. Clearing, grading, fill and foundation work in a landslide hazard area, or in an erosion hazard area disturbing 2,000 square feet or more, is not allowed from October 1 to April 1 unless the city grants a waiver backed by a geotechnical evaluation. That seasonal limit belongs in your schedule from the first week.

Check the hazard layers on the city’s online map before you buy or design. If your lot has critical areas, the approval also requires a notice recorded on title.

Remodel or rebuild: the 40 percent wall rule

Many older island houses sit closer to a lot line than today’s yards allow. Under MICC 19.01.050, a legally nonconforming house keeps that status through a remodel only while no more than 40 percent of the length of its existing exterior walls is structurally altered, counted cumulatively over five years. A wall segment counts once any part of it is removed down to no structure. Cross 40 percent and the whole house must meet current code, including setbacks. Raising the part of a house that already intrudes into a yard is treated as increasing the nonconformity.

If the plan you want alters more than 40 percent of the walls, price it as a rebuild to current setbacks and compare honestly. The wider decision is set out in remodel or tear down and rebuild.

Where and how to apply

Mercer Island runs its own permits through its Community Planning and Development department. The steps for a new house or a demo and rebuild:

1. Pre-application meeting. The city’s permit page calls it recommended, but MICC 19.15.050 requires one for any new or replacement house unless the city waives it. You meet a reviewer from each of five disciplines.
2. Intake screening. Mandatory for “substantial residential projects”: among them new houses, demo and rebuilds, additions of 500 square feet or more of footprint, and work that alters a critical area or its buffer. You upload the full application package and the city returns comments. It is not a pass or fail.
3. Application. Uploaded as PDFs through the city’s Permit Submittal Portal. A topographic survey is always required for a new house, along with a tree plan, stormwater plan, energy code worksheet and, where relevant, geotechnical and arborist reports.
4. Review and notice. Residential building permits carry public notice and a 30 day comment period. Review runs concurrently, but the permit cannot be finished before the comment period closes.

You can track permit status on MyBuildingPermit.com. Fees are based on project valuation, and the city does not offer paid expediting. Land use approvals such as a critical area review or shoreline permit are separate applications, though many can run concurrently with the building permit if you file the concurrent review form.

The building and energy codes sit in MICC Title 17. Which edition applies depends on when you apply, so confirm it at the pre-application meeting.

What realistic timelines look like

The city publishes review targets and describes them as estimates. As of September 2026: intake screening 2 weeks once all documents are in, routing 1 week after the application is complete, first review of a major building permit 4 weeks, each later review 2 weeks, and 1 week to prepare the approved permit. With one round of corrections that adds up to about 10 weeks of city time (2 + 1 + 4 + 2 + 1), before counting the completeness check or the time your team takes to answer comments.

The city code (MICC 19.15.040) also sets a decision deadline of 100 days from a complete application for a major building permit or a shoreline substantial development permit, and 65 days for a tree permit or shoreline exemption. Days spent waiting on the applicant are excluded, so the deadline is a ceiling on city time, not on the calendar.

For planning I use Seattle’s permit times as a benchmark, 6 to 10 months for a new house and about 2 to 3 months for a remodel, and 18 to 30 months from design start to move-in. Waterfront, steep slopes and big trees push toward the long end. The full sequence is in the custom home timeline, and budget ranges are in what it costs to build a house.

Where an architect fits

Every rule above can be checked before design starts. I put them on one site plan first, so the house is drawn inside the real envelope and the intake screening finds little to comment on. Piper Cole designs houses and administers construction on your behalf. We do not build.

Questions

How do I get a building permit on Mercer Island?

For a new house or a demo and rebuild, the city code requires a pre-application meeting (the city can waive it) and the city requires an intake screening before the building permit application. Applications are uploaded as PDFs through the city’s Permit Submittal Portal, and status can be checked on MyBuildingPermit.com. Residential building permits carry public notice with a 30 day comment period. Fees are based on project valuation, and paid expediting is not offered.

What are the residential zones on Mercer Island?

Single-family land is zoned R-8.4, R-9.6, R-12 or R-15. Minimum lot sizes are 8,400, 9,600, 12,000 and 15,000 square feet, with minimum widths of 60, 75, 75 and 90 feet. Gross floor area is capped at 40 percent of the lot, up to 5,000, 8,000, 10,000 or 12,000 square feet depending on the zone, as of September 2026.

What does R-15 zoning allow on Mercer Island?

R-15 requires a lot of at least 15,000 square feet and 90 feet wide. A house may have gross floor area up to 40 percent of the lot or 12,000 square feet, whichever is less, a maximum height of 30 feet above average building elevation, and lot coverage of 20 to 40 percent depending on slope. Confirm current figures with the city before designing.

What are the setback requirements on Mercer Island?

Front yards are 20 feet and rear yards 25 feet. Side yards must total at least 15 feet on lots 90 feet wide or less, or 17 percent of lot width on wider lots, with each side at least 5 feet or 33 percent of the total. On lots of 6,000 square feet or more, a side wall over 15 feet tall raises that side yard to 7.5 feet, and over 25 feet to 10 feet. Waterfront lots also have a 25 foot setback from the ordinary high water mark.

Do I need a permit to remove a tree on Mercer Island?

Generally yes, for any tree 10 inches or more in diameter, under MICC 19.10. Smaller trees are exempt unless they are exceptional trees, earlier replacement trees, or in a critical area or buffer. Removed trees must be replaced at 1 to 6 trees each depending on size. A new house on a lot of 6,000 square feet or more must also retain at least 30 percent of its trees of 10 inches or more.

How close to Lake Washington can I build on Mercer Island?

Structures must be at least 25 feet from the ordinary high water mark, in addition to the zoning setbacks. Hardscape and lot coverage are limited to 10 percent within 25 feet of the water and 30 percent between 25 and 50 feet. Under the shoreline code, a legally built house already within 25 feet may be remodeled or replaced as long as the nonconformity is not increased; zoning setbacks follow separate rules.

How long does a Mercer Island building permit take?

The city publishes targets rather than guarantees: 2 weeks for intake screening, 1 week for routing, 4 weeks for the first review of a major building permit, 2 weeks for each later review and 1 week for issuance. The code sets a 100 day decision deadline from a complete application, excluding time spent waiting on the applicant. As a benchmark, Seattle permits take 6 to 10 months for a new house and about 2 to 3 months for a remodel.

Can I keep my old setbacks if I remodel on Mercer Island?

Only within limits. A legally nonconforming house keeps its status if no more than 40 percent of the length of its existing exterior walls is structurally altered, counted over five years. Beyond that, the whole house has to meet current code, including setbacks. A rebuild must meet current setbacks from the start.

Authoritative sources

Regulatory requirements referenced in this guide are set by the official agencies below. Rules change, always confirm current requirements with the source:

• Seattle Dept. of Construction & Inspections (SDCI)