Bellevue ADU Permit Requirements, in Plain English

The short answer

Can I Build an ADU in Bellevue?

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. Bellevue calls a detached one a DADU. Backyard cottage, in law suite, granny flat, mother in law unit, guest house: if it is a second home on your lot, it is an ADU, and the rules on this page apply.

Bellevue rewrote its ADU rules in 2023 and again in 2025 and 2026. The old limits are gone. You no longer have to live on the property. The old 800 square foot cap is gone. Detached units are now allowed. If you read about Bellevue ADUs more than a year or two ago, most of what you read is out of date.

Today, on any lot where a single family home is allowed, you can build up to two ADUs in any mix of attached and detached. Each one can be up to 1,200 square feet. Most ADUs need no new parking space. This page walks through the current rules and the things that actually shape a project.

First, Make Sure Bellevue Is Your Permit City

Your mailing address and your permit city are not always the same. Some homes with a Bellevue address sit in unincorporated King County or in a neighboring city, each with its own rules. Before you plan anything, look up your parcel on the Bellevue Map Viewer. If the city holds your parcel, the rules on this page apply to you.

Your Zone Has a New Name

In 2025, Bellevue renamed every residential zone. Your plat, your old permits and most websites still use the old names. Here is the translation.

All of these zones allow ADUs. The most common single family zones in Bellevue are SR-3 (the old R-4), SR-4 (the old R-5) and LDR-1 (the old R-7.5).

Confirmed rules

Bellevue ADU Rules: What Is Confirmed

Zoning and How Many Units You Can Build

Bellevue allows up to two ADUs on any lot where a single family home or middle housing is allowed. You can mix them any way you want. Two attached, two detached, or one of each. There is no minimum lot size for an ADU.

Two exceptions. A lot created through a unit lot subdivision, where townhomes each sit on their own small lot, cannot add a new ADU. And a lot cannot have both an ADU and a home business that needs a Home Occupation Permit.

Size Limits

Each ADU can be up to 1,200 square feet. That is enough for a two bedroom, two bath home. On top of that, up to 300 square feet used for parking or unheated storage does not count toward the cap. So a 1,200 square foot cottage with a 300 square foot garage is allowed.

The city's planning director can approve a bigger unit in two cases: when the ADU takes up one whole floor of the main house, or when you are converting or adding onto a detached structure that already exists.

Height Limits

A detached ADU can be up to 24 feet tall, measured to the highest point. That leaves room for two stories. If you build the ADU on top of an existing garage or shed, the limit rises to 28 feet. An attached ADU follows the same height limit as your main house.

Setbacks

A setback is the minimum distance between a building and your property line. Bellevue uses two sets of setbacks for ADUs, and the difference matters.

An attached ADU follows the same setbacks as your main house. In most single family zones that is 20 feet from the street, 20 or 25 feet from the rear line, and 5 feet from each side line with a combined side total of 10 to 20 feet depending on zone. The large lot zones need 30 to 35 feet in front.

A detached ADU gets smaller setbacks. It follows Bellevue's middle housing standards, which were written to fit more homes on a lot.

There is one more break. If a detached ADU sits next to an alley, it can be built right at that alley line with no setback at all. Any setback that faces a street, access easement or private road has to be at least 10 feet.

If you are converting an existing detached garage that was legally built but sits closer to the line than today's rules allow, you can still turn it into an ADU. The conversion does not make it a code violation.

Owner Occupancy

Bellevue does not require you to live on the property. The city removed that rule in 2023, and state law now bans it. You do not have to live on site to build or rent an ADU.

Parking

Most Bellevue ADUs need no new parking space. There are two ways to qualify. If your ADU is under 1,000 square feet, not counting any garage, no parking is required. If your lot is within half a mile of a major transit stop, no parking is required at any size. Otherwise, one off street parking space per ADU. This is on top of whatever your main house already needs.

Renting It Out

You can rent your ADU, and you do not have to live next door. But an ADU must be rented for stays of 30 days or longer. Short term rentals, like nightly or weekly stays, are not allowed in an ADU in Bellevue.

Fees

The transportation impact fee for an ADU is $4,092 in 2026. An impact fee is a one time charge that helps the city pay for roads and growth. The ADU rate is half of what a new house pays. The fee goes up about 3 percent each January. On January 1, 2027 it becomes $4,215. You pay the rate in effect when your permit is issued, not when you apply.

School impact fees depend on which school district your lot sits in. Bellevue only collects them for lots in the Issaquah and Renton school districts, and an ADU pays half the regular rate. If your lot is in the Bellevue School District, which covers most of the city, we found no city collected school impact fee. Confirm your district before you budget.

Building permit and plan review fees are either a flat fee or a deposit the city bills against by the hour. The amounts are set in the city's annual fee ordinance. The city's Permit Fee Estimator gives a rough number. It is a planning tool, not a quote. The city's Fees and Payment page explains how billing works.

Water and sewer connection charges are set by Bellevue Utilities. The city does not publish a simple ADU figure, so ask Utilities directly at utilityreview@bellevuewa.gov before you set your budget.

How You Apply

You apply online through MyBuildingPermit.com. Which permit you need depends on what you are building.

• A new detached ADU, or a new addition that becomes an ADU, uses the New One or Two Family building permit. Bellevue treats an ADU like any other new home for permitting. Required documents include a site plan with addressing, floor plans, structural calculations, a storm drainage report, an arborist report, and a short construction stormwater form.
• An ADU created inside your existing house, like a basement conversion, uses the Single Family Remodel permit. It is a lighter review.

The city's permit pages for new construction and remodels list every required document.

Do I Need to Hire an Architect?

No, not by law. Washington state does not require a licensed architect for homes with one to four units, and an ADU falls under that rule. Bellevue does not add an architect requirement of its own. A homeowner, a designer or a design build firm can prepare the drawings. You can read the state rule yourself at RCW 18.08.410.

That said, Bellevue asks for more than many cities. A new ADU permit requires structural calculations, a storm drainage report and an arborist report with the application. Your site plan has to show property lines, setbacks, lot coverage and every significant tree correctly. Your floor plans have to meet the building code for things like egress windows, ceiling height and fire separation. Drawings that miss these come back with comments, and every round of comments adds time.

Some parts of the project do need a licensed professional. A geotechnical report must come from a licensed engineer. Retaining walls over 4 feet and unusual structural work, like cutting new openings into a foundation, usually need an engineer's stamp. If your lot is flat and the design is simple, you may never need one. If your lot is sloped, plan on it.

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. Bellevue has more hills, more streams and more big trees than most Eastside cities, so these come up often. The sooner you know which ones apply to your lot, the fewer surprises you get later.

Slopes and Soil: When You Need a Geotechnical Report

A geotechnical report is a soil and slope study done by a licensed engineer. It tells the city whether the ground can safely hold a new building. In Bellevue, the critical areas code is what triggers one. Critical areas are places the city treats as sensitive, like steep slopes, unstable soil, streams and wetlands. Bellevue's rules are in LUC 20.25H.120.

Here is what the city looks for:

• Steep slopes. Ground that drops 40 percent or more, rises at least 10 feet, and covers more than 1,000 square feet. The city puts a 50 foot buffer at the top of the slope and a 75 foot setback at the bottom. Nothing gets built inside those zones without an engineer's study.
• Landslide hazard. Ground that slopes 15 percent or more with more than 10 feet of rise, plus a sign of past trouble like old slide deposits, seeps, or a stream cutting into the bank. Same 50 foot buffer at the top. The setback at the bottom is set by the engineer's study.
• Earthquake soil. Known faults, or soil rated moderate to high for liquefaction on the state map. Liquefaction means loose, wet soil can act like liquid during an earthquake.
• Old coal mines. Parts of Bellevue, especially Newcastle and the Coal Creek area, sit over abandoned mines. The city maps these and may ask for a study.

Those buffers can shrink. If a licensed engineer or geologist studies the site and finds low risk, the city can cut a buffer down to 15 feet. Going closer than that, or building on the hazard itself, means a deeper study and a critical areas permit.

A 15 percent slope is not steep. It is a rise of 15 feet over 100 feet of distance, the kind of grade you barely notice walking. A lot of Bellevue lots have at least a strip of it. If yours does, plan for a soil report. Bellevue does not publish how long one takes or a list of approved firms. Get schedules from local firms before you commit to a start date.

Streams and Wetlands

Bellevue has a lot of small streams, many of them piped underground. Each one comes with a buffer, which is a no build zone on each side. Stream buffers run from 50 to 150 feet depending on the stream type. Wetland buffers run from 40 to 225 feet depending on the wetland category. A stream you have never noticed at the back of your lot can take a big bite out of where an ADU fits. The buffer widths are in LUC 20.25H.035.

One more thing. When Bellevue calculates your lot coverage, it subtracts critical areas and stream buffers from your lot size first. So a buffer costs you twice: once in where you can build, and once in how much you can build.

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

Lot coverage is how much of your lot is covered by buildings. For a detached ADU, Bellevue caps it at 40 percent of the lot in the LL-1 through SR-3 zones, and 45 percent in SR-4 and the LDR and MDR zones. That count includes your house, your garage, any sheds, and the new ADU. Here is how the math works.

In that example, a 1,200 square foot single story cottage does not fit. A two story design with a 600 square foot footprint does, with room to spare. A basement conversion adds no new footprint at all.

Bellevue also caps impervious surface, which counts roofs plus driveways, walkways and patios. For a detached ADU that cap runs from 50 to 65 percent depending on zone. A new driveway to a backyard cottage can push you over it even when the building itself fits. Permeable paving, which lets rain soak through, can help.

County records are often fuzzy about your existing footprint. They may not separate your basement from your main floor, or show the true outline of your house. Records will not settle it. A real site plan or survey will.

Trees

This is the complication Bellevue homeowners most often do not see coming. Bellevue protects trees harder than most cities, and the rules got tighter on January 1, 2026.

A significant tree is any tree 6 inches across or more, measured 4 feet above the ground. That is not a big tree. A landmark tree is generally 24 inches across or more. Removing either one needs a permit, with a small yearly allowance. New construction requires an arborist report with the permit application, and the city sets a minimum tree density your lot has to keep through retention and new planting.

In practice, this means a big tree in the middle of your backyard can decide where the ADU goes, not the other way around. Walk your lot and count anything over 6 inches across before you fall in love with a location. The city's tree removal page covers the basics.

Fire Separation for Attached Units

Once part of your house becomes its own legal home, the two units need a fire rated wall and floor between them. A fire rated assembly is built with materials that slow a fire down, so a fire in one unit cannot spread fast into the other. Any door between the two units has to be a fire rated, self closing door. For a basement ADU, that usually means upgrading the ceiling between the basement and the main floor.

Egress and Basement Conversions

Egress means a safe way out in an emergency. Every bedroom in an ADU needs its own emergency escape window, and the unit needs a clear path to an outside door. In a basement, that often means cutting a larger opening into the foundation and digging a window well outside. This is real construction work, not paperwork.

Basement conversions have a few other checks too:

• Ceiling height. The space needs at least 6 feet 8 inches of ceiling height. Measure it before you plan anything else.
• A full kitchen. A kitchenette with a sink and a fridge does not count. A legal ADU kitchen needs a permanent cooking appliance, plus the gas or electrical work to run it.
• The separate entrance. Bellevue no longer bans a second front door. A walkout or outdoor stairway still has to be checked against your foundation.

Stormwater, Grading and Retaining Walls

Stormwater is rain that runs off roofs and paving instead of soaking into the ground. Bellevue adds stormwater requirements once a project creates or replaces 2,000 square feet or more of hard surface. Past that line, you need a plan to manage that runoff on your lot. More rules kick in at 5,000 square feet. Every new ADU permit needs a storm drainage report either way.

The same 2,000 square foot line triggers a separate clearing and grading permit. So does moving more than 50 cubic yards of dirt, clearing more than 1,000 square feet of land in a year, or doing any work inside a critical area or its buffer. A retaining wall over 4 feet tall, measured from the bottom of the footing, needs its own permit, and a wall over 30 inches cannot sit inside a required setback.

Many ADUs add somewhere around 500 to 1,700 square feet of new hard surface, which stays under the line. A bigger unit with a new driveway or long walkway can cross it. A real site plan with the driveway and walkways drawn in is how you know for sure.

Other Items to Confirm

• Water and sewer. Bellevue allows a single water meter and sewer line to serve the lot in many cases, but the city does not publish a clear ADU rule. Ask Utilities at utilityreview@bellevuewa.gov what your lot needs and what it costs.
• Pre approved plans. Bellevue is working on a set of pre approved detached ADU designs that would speed up permitting. As of October 2026 they are not available yet. The city's pre approved plans page has the latest.

Timeline

How Long Does a Bellevue ADU Permit Take?

Bellevue is one of the few Eastside cities that publishes review targets. They are targets, not promises, and the city says actual times vary. But they give you something real to plan around.

Most projects go through two or three rounds. A clean, complete first submittal is the single best way to keep that number low. The targets are posted on the city's permit review timelines page.

Here is the order things happen in, and where the delays come from.

1. Lot review. Your property lines, hard surfaces, slopes, streams and significant trees get measured on the ground, not pulled from records.
2. Design. Unit sizes and locations get tested against the setbacks, the trees, the slope and your yard. You should see options before anything is drawn for the city.
3. Reports, if required. An arborist report is required for every new ADU. A geotechnical report is required on sloped or hazard mapped lots. Neither has a published turnaround, so get real schedules from local firms before you commit to a date.
4. Permit submittal. The application goes in online through MyBuildingPermit.com with the site plan, floor plans, structural calculations, drainage report and arborist report.
5. City review. The city reviews the plans and sends comments. The design team answers them and resubmits as needed.
6. Permit issued. Once the city approves, construction can start.

The biggest swing factors in Bellevue are trees and slopes. A flat lot with no big trees can move fast. A sloped, wooded lot may wait on two separate reports before the design can be locked. Settling those early is the best way to keep your timeline from slipping.