Mercer Island ADU Permit Requirements, in Plain English

The short answer

Can I Build an ADU in Mercer Island?

An ADU is an accessory dwelling unit. It is a second, smaller home on the same lot as your house. It can be attached, like a basement apartment or an addition. Or it can be detached, like a backyard cottage. Backyard cottage, in law suite, granny flat, mother in law unit, guest house: if it is a second home on your lot with its own kitchen, bathroom and entrance, it is an ADU, and the rules on this page apply.

On Mercer Island, you can build up to two ADUs per lot, attached or detached. Each one must be at least 220 square feet and can be up to 1,000 square feet. You do not have to live on the property. That is more room than the island allowed a year ago, when the limit was one ADU of 900 square feet.

First, Make Sure Mercer Island Is Your Permit City

Your mailing address and your permit city are not always the same. Mercer Island is its own city with its own planning department. Check your parcel against the city zoning map before you plan anything. If the city holds your parcel, the rules on this page apply to you.

Confirmed rules

Mercer Island ADU Rules: What Is Confirmed

Zoning and How Many Units You Can Build

Mercer Island allows up to two ADUs on a lot, attached or detached. The city's rules for middle housing, which means small multi unit buildings like duplexes, share some of the same lot limits, so the total number of units on your lot depends on how you combine them. Ask the city to confirm your total before you design.

Size Limits

An ADU can be 220 to 1,000 square feet. Garage area does not count toward the 1,000. A 1,000 square foot unit is enough for a two bedroom home. The code does not say whether the 1,000 is measured inside the walls or outside them. Ask the planner before you draw to the limit.

Height Limits

The limit is 30 feet, measured from the average building elevation to the highest point of the roof. Small items like chimneys can go up to 5 feet higher. That is taller than the state minimum of 24 feet, and it leaves room for two stories. The downhill face of a building is also held to 30 feet, which matters on a sloped lot.

Setbacks and Yards

A setback, or yard, is the minimum distance between a building and your property line. Mercer Island has no separate setback table for ADUs. A new ADU follows the same rules as your main house. In the R9.6 zone, that is 20 feet in front and 25 feet in back.

Side yards work differently. On a lot wider than 90 feet, the two side yards together must equal at least 17 percent of the lot width. Each side must be at least 5 feet, or a third of that total, whichever is more. On lots of 6,000 square feet or more, the code also sets 7.5 foot and 10 foot side yards based on wall height.

Two details matter here. First, the city measures lot width between the midpoints of the two side lot lines, and lot depth from the front midpoint to the rear midpoint. On a wide, shallow lot, that can push the side yards out and leave a narrow strip to build in. Second, the front of your lot is the line on the street, which is not always the side you think of as the front.

An accessory building cannot sit in a required yard. The one exception is a small shed of 200 square feet or less and 12 feet tall or less in the rear yard. That does not help a cottage.

A building counts as detached only if it shares no wall with the house and sits more than 5 feet away. At 5 feet or less, the city treats it as attached. The city can approve a setback deviation, which is a small exception, but the decision takes 65 days and the rules for it are narrow.

Owner Occupancy

Mercer Island does not require you to live on the property. You can live in the ADU and rent the main house, or the other way around. Older city forms asked for an owner occupancy affidavit. The January 2026 ADU form does not. If a planner asks for one, get the current position in writing.

Parking

Lots over 6,000 square feet need 2 off street spaces per ADU. Smaller lots need 1 per unit. A lot within half a mile walk of a major transit stop, like the light rail station, needs none. The city measures that walk along real routes, so ask the city to check your address. Your main house must also still meet its own parking rule.

Design Match

A new detached ADU must be designed to fit the main house, including its roof pitch, siding and windows. This rule applies only to ADUs. It may conflict with state law, so ask the planner how it is applied before you pick finishes.

Lot Coverage and Hardscape: The Math That Decides How Big You Can Go

Lot coverage is how much of your lot is covered by buildings and driving surfaces. It counts your house, garage, eaves, the new ADU and your driveway. It does not count decks and porches. The cap depends on how steep your lot is.

In that example, a full size 1,000 square foot cottage fits with room to spare. Slope is the swing factor. County records do not show slope, so a survey settles it.

Mercer Island also has a separate hardscape cap. Hardscape means stairs, walkways, decks and patios. The cap is 9 percent of net lot area, which is 1,080 square feet on that same 12,000 square foot lot. Driveways and buildings are not counted here. Large existing decks can use up most of this cap, and any new patio or walkway for the ADU adds to it. Whether elevated decks count, and whether an older deck is legal, is something to confirm with the city.

There is no percentage cap on total impervious surface for a house lot. Impervious surface means roofs, driveways and paving that water cannot soak through. It still matters, because it feeds into the stormwater review below.

Fees

Mercer Island's fee schedule changed on August 1, 2026. The city bills staff time at $182 an hour. The table below shows the charges that clearly apply to a 1,000 square foot ADU, from the city fee schedule. These are city and utility charges only. They do not include design, survey, engineering, consultant reports or construction.

Then there is a second list. These are charges the fee schedule shows but the city has not confirmed in writing for ADUs. Some conflict with the city's own code. Ask the city about each one early, because together they can move your total investment by tens of thousands of dollars.

The first three open items add up to about $21,300. With the affordable housing fee, the total reaches about $46,300. That is simple arithmetic on published numbers, not a prediction. Treat the affordable housing question as the largest single unknown on this page.

How You Apply

An ADU takes two separate applications. The first is an ADU land use application, called a Type II review. The second is the building permit. The city may also ask you to file them at the same time.

For the land use application, the city asks for:

• The development application form and a project narrative.
• A code compliance matrix, which is a checklist showing how your plan meets each ADU rule.
• A title report less than 30 days old.
• A notarized affidavit of agent authority, unless the city waives it.
• A plan set that follows the city's plan set guide.

For the building permit, an intake screening request comes first. It is required for new homes, for projects that add 500 square feet or more of impervious surface, and for any project that touches a critical area or buffer. After that come the permit application, a site development worksheet, a small project stormwater report, a water meter sizing worksheet, a fire area and valuation form, a tree inventory form and the energy code forms.

The city says a complete survey is always required for new residential construction. Plan on paying for one. The city's pages list several ways to submit, including an online portal and MyBuildingPermit.com. City Hall was shown as closed on the permit pages in October 2026, so confirm how to file before you submit anything. Details are on the city's new home permit page and the ADU form, LU 16.

Watch for these

What Can Complicate Your Project

None of these usually kill a project. They change where the unit goes, what it costs, or how long it takes. The sooner you know which ones apply to your lot, the fewer surprises you get later.

Stream Buffers

A stream buffer is a no build zone around a creek or ditch. Mercer Island's most common small stream, called a Type Ns watercourse, has a 60 foot buffer. Buildings must also stay 10 feet back from the edge of that buffer. In some cases that drops to 5 feet, but only if the stream has no priority habitat, and you add a split rail fence and survey markers.

Here is the part that surprises people. The stream does not have to be on your lot. If a neighbor's stream sits close to your property line, its buffer can reach onto your land and take a big bite out of your yard. New buildings, driveways and paving cannot go inside the buffer. A new detached ADU does not fit any of the exceptions.

If your lot is touched by a buffer, plan on these steps:

• Critical area review, level 1. The city checks where the buffer sits. It is reviewed along with your building permit, and it costs $1,456.
• A notice on your title. The city wants proof that a notice about the critical area is recorded on your property before it approves anything.
• Field checks. The city may require a specialist to confirm where the stream actually is.
• Tree rules. A stream buffer counts as a critical tree area, so even small trees inside it need a permit.

You can ask to shrink or average the buffer down to 75 percent of its width, with mitigation. That moves you into critical area review, level 2. It is a much heavier process. It adds a pre application meeting, public notice, a 30 day comment period, a critical area study, an environmental checklist and likely a peer review. Consultant costs are yours to pay. Most projects are better off moving the ADU to a spot outside the buffer.

One more thing. State law gives ADUs strong protection, but it carves out lots with critical areas or their buffers. On a lot with a mapped buffer, the city has more room to apply its own limits.

Slopes and Soil: When You Need a Geotechnical Report

A geotechnical report is a soil and slope study done by a licensed engineer. Mercer Island requires a critical area study when work happens in a landslide hazard area, a seismic hazard area, or the buffer around one. The city can waive it for minor work or when your site is not in a hazard area.

The city maps these areas, but its own tip sheet warns that the maps are not precise enough to find the exact edge in the field. Check yours on the city's geological hazards page. A few things to know:

• Distance matters. Landslide buffers run from 25 feet for shallow slides to 75 feet for deep seated ones. A lot that sits well outside every mapped layer usually does not need a report.
• Digging can change the answer. A footprint, grading or utility trench that reaches toward a hazard edge can flip a lot from no report to report required.
• Seasonal limits. Work on erosion and landslide hazard sites is limited from October 1 to April 1. That applies only to those mapped sites.

The city does not publish how long a soil report takes, and it does not publish a list of approved firms. It points to King County's consultant list instead. If you are unsure, a short call with the city's on call planner can save you from paying for a report you do not need.

Stormwater

Stormwater is rain that runs off roofs and paving instead of soaking into the ground. Mercer Island requires you to manage that runoff on your own lot when a project adds 500 square feet or more of new impervious surface. A 1,000 square foot ADU crosses that line by itself. You then complete a small project stormwater report using low impact methods, such as rain gardens or downspout dispersion. These are details in the city's stormwater standards.

Other thresholds sit higher. At 2,000 square feet of new and replaced hard surface, more requirements apply. At 5,000 square feet, treatment and flow control rules start. Many ADUs, once you add parking and a walkway, land somewhere in the middle. A real site plan with the driveway and walkways drawn in is how you know for sure.

Detention is the wild card. Detention means holding runoff in a tank or vault and releasing it slowly. The city can require it when low impact methods will not work and your lot drains toward a stream or a system with limited capacity. If it applies, you either build a system or pay the fee in lieu shown above. Parts of the island are mapped as poor for infiltration, so ask the city early which side of the line your lot falls on.

Fire Sprinklers and Water Service

The city's 2026 forms point toward automatic fire sprinklers in a new detached ADU. The exact rule for ADUs is not confirmed, so ask the fire marshal. Fire review runs through Eastside Fire and Rescue, not a city fire department. If sprinklers are required, they push your water service up to at least a 1 inch line. That raises the connection charges in the fee table. Your site also needs fire access: a road within 150 feet of the first floor wall, 20 feet wide, with a turnaround for long dead ends.

Fire Separation and Basement Conversions

If your ADU is a basement or an attached space, the two units need a fire rated wall and floor between them. That usually means upgrading the ceiling over a basement unit. Every bedroom needs its own emergency escape window. In a basement, that often means cutting a bigger opening into the foundation and digging a window well. This is real construction, not paperwork. Confirm the details with the city before design starts.

Trees

Mercer Island asks for a tree inventory with your building permit, and for an arborist report or tree plan if the city requires one. It also looks at the roots of neighbor trees whose branches cross your lot line. Fees for tree retention and replacement apply only if your project triggers them.

Other Items to Confirm

• Old houses. Many Mercer Island homes were built before today's yard rules. A house that sits inside a current setback is often legal because it came first. That does not block a separate ADU, but it usually means the house cannot be extended toward that line.
• Shared water and sewer. We could not confirm whether an ADU may share your house's water meter and side sewer line.
• Short term rentals. We did not review the city's short stay rules. State law lets cities limit ADUs for rentals by the night. If you plan to rent that way, confirm with the city before design.
• Renting out the main house. The city passed a 2026 ordinance on tenant protections that we did not read. If you plan to rent your main house, ask how it applies.

Timeline

How Long Does a Mercer Island ADU Permit Take?

Mercer Island does something many cities do not. It publishes review targets. It does not publish average or actual review times, and it offers no paid rush option. So treat the targets below as the best case, not a promise. You should also expect several rounds of comments. The city's review timelines page lists them.

The law also sets outer limits. The city has 65 days to decide an ADU land use application once it is complete. A smaller building permit also has 65 days, and a major one has 100 days. Whether a detached ADU counts as major is something to ask the city. Adding the targets together, a first land use pass takes about six weeks, and the building permit follows with more rounds. That total is our own adding up, not a city number.

Here is the order things happen in:

1. Lot review and survey. Your property lines, slopes, buffers and existing buildings get measured on the ground, not pulled from records.
2. Design. Unit size and location get tested against the yards, the buffer and your driveway. You should see options before anything is drawn for the city.
3. Critical area and soil questions. If a buffer or hazard layer touches your lot, those get settled early, because they can move the whole plan.
4. ADU land use application. This is the Type II review. It has no public notice, no comment period and no hearing.
5. Building permit. Intake screening, routing, plan review and correction rounds follow.
6. Permit issued. Once the city approves, construction can start. The water tap happens about two weeks after the parts inspection.

The biggest swing factors are a stream buffer, a soil report and the open fee questions. Settling those first is the best way to keep your timeline and your total investment from drifting.