Short answer: the zone name gives you the minimum lot size, and yards, floor area, height and lot coverage then shape the house. Trees, steep slopes and the lake add rules on top.

This page is for owners planning a remodel or rebuild of $750,000 or more. The figures come from Title 19 of the Mercer Island City Code (MICC) and the city’s own pages, read on 13 October 2026. The online code is codified through Ordinance 26C-03 (3 March 2026). Ordinance 25C-27, adopted 18 November 2025, makes the city’s interim housing and permit rules permanent and is still listed as not yet codified. The city’s hearing notice for it names MICC 19.01.050, 19.02.020, 19.10.060 and 19.16.010 among the sections it amends without describing the changes. I could not read the redline, so confirm any number with the city. The permit steps are in my Mercer Island building permit guide.

The four single-family zones and how to find yours

Title 19 is the city’s Unified Land Development Code, the land use code behind every number here. To find your zone, open the city’s online Mercer Island zoning map, search the address and read the label on your parcel. The city calls its zoning page a quick reference and sends you to Title 19, which is on Municode.

Source: MICC 19.02.020(A) and (D). Whichever floor area figure is less applies. Lots created before September 28, 1960 are not held to the minimum area. The minimum-area test leaves out any part of the lot that is a street, and the city page also excludes shorelands and vehicular access easements, so have a waterfront survey checked. A lot of 7,500 square feet or less may use the lesser of 3,000 square feet or 45 percent. Coverage and hardscape use net lot area, which subtracts vehicular access easements (MICC 19.16.010).

The 40 percent figure decides first. The fixed cap takes over only 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.

Setbacks and the side yard rule

The code calls setbacks yards. These minimums in MICC 19.02.020(C) apply in all four zones.

• Front yard: 20 feet. Rear yard: 25 feet.
• Side yards, total: at least 15 feet on a lot 90 feet wide or less, and at least 17 percent of the width on a wider lot.
• Side yards, each: at least 5 feet or 33 percent of that total, whichever is greater.
• Tall walls: on a lot of 6,000 square feet or more, a side wall over 15 feet (a gable end over 18 feet) needs 7.5 feet on the side that touches a neighbor, and a wall over 25 feet needs 10 feet. Height is taken from existing or finished grade, whichever is lower.

A two story side elevation clears 15 feet, so on a narrow lot the working side yard is usually 7.5 feet and not 5.

Porches, chimneys, window wells and unroofed stairs and decks may project 3 feet into a required yard and eaves 18 inches, but nothing projects into the minimum side yard beside a neighbor. The code official may approve a setback deviation (MICC 19.02.020(C)(4)).

Easements add a layer. The code bars structures on a vehicular access easement and wants 5 feet clear of its edge (MICC 19.02.020(H)(1)). The city’s quick-reference page lists 10 feet for rights-of-way and access easements, so ask which figure the city applies to your lot.

Floor area, height and lot coverage

Floor area is measured to the outside faces of the walls. It includes garages, detached accessory buildings over 120 square feet, the part of a basement that projects above grade, staircases and covered upper decks. First-level covered patios and uncovered upper decks are left out. The part of a room with a 12 to 16 foot ceiling counts at 150 percent and the part above 16 feet at 200 percent, so a 20 by 20 foot room with a 20 foot ceiling is 400 square feet on the plan and 800 against your cap.

Height is 30 feet above the average building elevation to the highest point of the roof. On a sloping lot the downhill facade may also not exceed 30 feet, measured from the lower of existing or finished grade at the furthest downhill point to the top of the wall that carries the roof.

The average building elevation is the length-weighted average of the ground at each wall’s midpoint. Take a 60 by 40 foot house with grade at 100.0 at the front wall, 94.0 at the rear and 97.0 at each side. Then (60 x 100.0) + (60 x 94.0) + (40 x 97.0) + (40 x 97.0) = 19,400, and 19,400 / 200 feet of wall = 97.0, so the roof peak may reach 127.0. If the ground at the furthest downhill point is 92.0, the top of the downhill wall may not pass 122.0 (formula: MICC 19.02.020(E)(4)).

Lot coverage is the roof outline, eaves included, plus driving surfaces. It steps down with slope, which the code calculates as the highest elevation minus the lowest, divided by the shortest horizontal distance between them. Twelve feet of fall over 60 feet is 20 percent.

Source: MICC 19.02.020(F), which applies to remodels as well as new houses. Coverage means house, driving surfaces and accessory buildings. Hardscape is capped at 9 percent of net lot area in every band, with a small-lot allowance at 8,400 square feet or less.

Up to 9 percent of the net lot may be hardscape (walkways, patios, decks, stairs) inside the landscaping area, and driveways may not. A pool cannot sit in a front yard or within 5 feet of a neighbor (MICC 19.02.070).

Worked example: a 15,000 square foot R-15 lot

Take a lot 100 feet wide and 150 feet deep, flat, with no easements, critical areas or shoreline. It meets R-15’s 15,000 square feet, 90 feet of width and 80 feet of depth.

The yards leave an 8,715 square foot envelope, and coverage cuts the house, driveway and outbuildings to 6,000 of it. Detached accessory buildings together may use at most 25 percent of the floor area, or 1,500 square feet here. Floor area is a separate limit, because a second story adds floor without adding coverage.

Slope changes the answer. If the lot falls 12 feet over 60 feet, that is 20 percent and coverage becomes 15,000 x 0.35 = 5,250 square feet. At 30 to 50 percent it is 4,500. Read literally, hardscape comes on top of coverage, so the flat lot could carry 6,000 + 1,350 = 7,350 square feet of hard surface, about 49 percent. That is my reading, so ask the city how it counts a terrace.

Trees

The tree chapter, MICC 19.10, shapes where a house goes as much as the yards do. A permit is needed to remove a tree, except small trees under 10 inches that are not exceptional and sit outside critical areas. A new house, or an addition or remodel adding more than 500 square feet of floor area, on a lot of 6,000 square feet or more must keep at least 30 percent of the trees 10 inches or larger over a rolling five years. If the lot has 20 such trees, six stay. Exceptional trees of 24 inches or more must be kept unless a narrow exception applies. Removed trees are replaced at 1, 2, 3 or 6 trees by size, planted from October 1 to April 1.

Steep slopes and the wet season

The city says it is very common for an island project to touch a mapped geologic hazard; check the hazard layer on its online map. A steep slope is 40 percent or more, measured as rise over any 30 foot run. Its buffer sits at the top and toe and equals the slope’s height, up to 75 feet. Shallow landslide hazard areas carry a buffer of at least 25 feet and deep-seated ones 75 feet. Building is allowed with a critical area study, and the city asks for a geotechnical report from a Washington licensed engineer.

The detail that moves schedules: land clearing, grading, filling and foundation work in a landslide hazard area, or in an erosion hazard area when 2,000 square feet or more is disturbed, is not permitted from October 1 to April 1 (MICC 19.07.160(F)(2)). The code official may grant a waiver backed by a critical area study.

Lake Washington lots

The code defines shorelands as Lake Washington and the land within 200 feet of its ordinary high water mark (OHWM). Where the shoreline chapter, MICC 19.13, conflicts with other code, it governs inside the shorelands. Its Table C sets these standards:

• Setback: all structures, including fences over 48 inches, stay 25 feet from the OHWM and also meet the zoning yards.
• Height: the zoning limit applies, capped at 35 feet.
• Coverage near the water: hardscape and lot coverage are limited to 10 percent from 0 to 25 feet from the OHWM and 30 percent from 25 to 50 feet.
• Native planting (MICC 19.13.050(K)): new floor area, coverage and hardscape of 500 square feet or more, counting the five years before the application, requires native vegetation over 50 percent of the 20 foot vegetation area. At 1,000 square feet it is 75 percent.

On 100 feet of straight shoreline, the first 25 feet is 100 x 25 = 2,500 square feet, so 10 percent is 250. The band from 25 to 50 feet is 2,500 square feet and 30 percent is 750. The 20 foot vegetation area is 2,000 square feet, so 50 percent is 1,000 and 75 percent is 1,500. That reads the percentages as shares of each band, my reading of Table C. Your shoreline is rarely straight, so the survey sets the real figures.

A legally built structure within 25 feet of the water may be remodeled and completely replaced if the nonconformance does not increase, and an expansion is allowed when the new part complies (MICC 19.13.020). An owner-occupied single-family house up to 35 feet high is exempt from the shoreline substantial development permit, but the city still requires a shoreline exemption application. See my shoreline exemption guide, waterfront remodel guide and waterfront architect page.

Remodeling a house that breaks today’s rules

Under MICC 19.01.050(D), a legally nonconforming detached house keeps that status only while no more than 40 percent of the length of its existing exterior walls is structurally altered, counted over any five years, which takes in work done in the five years before your permit. A wall segment counts as altered once any part of it is completely removed. The code’s example is a 50 foot cube with 200 feet of wall, where 40 percent is 80 feet. Roof replacement and interior remodeling do not count, and no alteration may increase an existing nonconformity or add a new one. The full count is on my Mercer Island remodel architect page, and other cities are compared in when a remodel counts as a new house. Past 40 percent, every number here applies to the whole house.

ADUs, middle housing and design review

ADUs. A lot that meets the minimum size may have up to two accessory dwelling units, attached or detached, of 220 to 1,000 square feet excluding garage area, under the same development standards as the main house (MICC 19.02.030). An attached ADU can raise the floor area allowance by up to 5 percentage points, but the total may not exceed 4,500 square feet or 45 percent. That helps a small lot, not a 15,000 square foot lot already allowed 6,000.

Middle housing. MICC 19.02.025 allows two units per lot in all four zones, and four in cases the code lists, under the same regulations as a detached house.

Design review. I find none for a single-family house. The design standards chapter, MICC 19.12, applies to regulated improvements, a term the code defines to exclude single-family dwellings, middle housing, ADUs and their accessory buildings.

I have been an architect since 1996, and I put these rules on one site plan before design starts. Piper Cole designs and administers construction on your behalf and does not build.

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