Issaquah ADU Permit Requirements, in Plain English

The short answer

Can I Build an ADU in Issaquah?

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. Issaquah 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.

Issaquah rewrote its ADU rules in 2023 and again in 2025 to match new state law. Owner occupancy is gone. The old one ADU limit is gone. The city no longer requires a parking space for an ADU. If you looked into this a few years ago and got told no, the answer has changed.

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 1,000 square feet. This page walks through the current rules and the things that actually shape a project.

First, Make Sure Issaquah Is Your Permit City

This matters more in Issaquah than almost anywhere on the Eastside. A large share of homes with an Issaquah mailing address are not in the city at all. Mirrormont, Tiger Mountain, May Valley, Preston, the Hobart Road corridor and much of the land south and east of town sit in unincorporated King County. The county has its own ADU rules, and on rural zoned land a detached ADU needs a lot of several acres. The city rules on this page do not apply there.

Before you plan anything, look up your parcel on King County Parcel Viewer. If the jurisdiction line says Issaquah, this page is for you. If it says King County, the county permit office is your starting point. Klahanie joined the city in 2016, so most Klahanie homes now follow the Issaquah rules.

Which Zone You Are In

Issaquah has four main single family zones outside its planned neighborhoods. Your zone sets your setbacks and how much of your lot can be covered. The city's maps page links to the interactive zoning map that shows which one you are in.

Issaquah Highlands and Talus use their own urban village zones. ADUs are allowed there too, but each neighborhood has an Architectural Review Committee that can set stricter setbacks and design rules than the city. If you live in either one, plan on two reviews: the committee first, then the city.

Confirmed rules

Issaquah ADU Rules: What Is Confirmed

Zoning and How Many Units You Can Build

Issaquah allows up to two ADUs with any single family home, as long as the lot meets the minimum lot size for its zone. You can mix them any way you want. Two attached, two detached, or one of each. The code's own example is one ADU inside the house and one above the garage.

If your lot is smaller than the zone minimum but was legally created, you can still build up to two ADUs. The city treats it as a legal nonconforming lot. With a single family home, ADUs do not count toward the lot's density. If your lot has a duplex, triplex or other middle housing instead of a single house, an ADU does count, which can limit how many you get.

Size Limits

Each ADU can be up to 1,000 square feet of living space. That is enough for a two bedroom, two bath home. Garage space, storage sheds and decks do not count toward the cap, unless that space is where the ADU itself is going.

One rule is easy to miss. Your main house has to be at least 25 percent larger than your biggest ADU, measured by gross floor area. For a 1,000 square foot ADU, the main house needs to be at least 1,250 square feet. Most Issaquah homes clear that easily. A small older cottage might not.

An ADU can go over 1,000 square feet in two cases: when you carve it out of your existing house without expanding the building, or when you convert an existing garage or outbuilding without expanding it. Small changes needed to make it a working home, like a new door, are fine.

Height Limits

An ADU follows the same height limit as the main house in your zone. In SF-E, SF-S and SF-SL that is 30 feet. In SF-D it is 25 feet or two stories, whichever is lower. Issaquah measures height from average grade to the midpoint of a pitched roof, so a two story detached cottage fits comfortably in most zones.

Setbacks

A setback is the minimum distance between a building and your property line. Issaquah does not give ADUs a special reduced setback the way some neighboring cities do. Attached or detached, an ADU follows the setbacks for the main house in your zone.

Two breaks help. If your rear lot line touches a public alley, a new ADU needs no rear setback at all, unless the city plows that alley. And a garage facing an alley only needs enough setback so the alley width plus the setback adds up to 24 feet. On a corner lot, both street sides count as front.

If you are converting an existing garage or shed that was legally built but sits closer to the line than today's rules allow, you can still turn it into an ADU. The building official has to find the structure sound, and any part that changes use has to meet today's building code.

Hard Surface Limits

Issaquah does not use a simple lot coverage number. It caps impervious surface, which is anything rain cannot soak through. That means roofs, driveways, walkways, patios, decks and sport courts, all added together. The cap is 40 percent of the lot in SF-S, 50 percent in SF-SL and SF-D, and 30 percent in SF-E. This is the rule that most often decides how big a backyard cottage can be. The math is in the next section.

Owner Occupancy

Issaquah does not require you to live on the property. The city dropped that rule when it rewrote the code in 2023, and state law now bans it. You can build an ADU and rent it out without living next door. The city also cannot stop you from selling the ADU separately as a condominium just because it started as an ADU.

Parking

No parking is required for an ADU anywhere in Issaquah. The city's parking table sets the minimum at zero and the maximum at one space per ADU. Tandem parking, where one car parks behind another in the driveway, counts if you choose to add a space. Your main house keeps whatever parking it already has.

Renting It Out

You can rent your ADU, and you do not have to live on site. The ADU code itself does not set a minimum stay. Issaquah regulates short term rentals under its business license rules, so check with the city before you plan on nightly or weekly stays.

Fees

This is where Issaquah stands out. Most Eastside cities charge impact fees on an ADU, which are one time charges toward roads, parks, schools and fire stations. Issaquah exempts ADUs from all four. The city's traffic, park, school and fire impact fee ordinances each say a permit for an accessory unit is not assessed the fee, and the city's own impact fee page lists ADUs as one of the two most common exemptions. On a new house those fees add up to tens of thousands of dollars. On an ADU they are zero.

You still pay building permit, plan review and site work fees. Issaquah bills these by project value and by the hour for some reviews, and invoices them after your application is accepted. The city's single family fee estimator gives a rough number. It is a planning tool, not a quote.

Water and sewer connection charges are separate. Most of the city is served by Issaquah's own utility. Issaquah Highlands, Klahanie and some other areas are served by Sammamish Plateau Water. Which one you have decides what you pay to connect. See the utilities note below.

How You Apply

You apply online through MyBuildingPermit.com. The city does not accept paper plans. Which permit you need depends on what you are building.

• A new detached ADU uses the Single Family Residential, New Construction, Detached Accessory Dwelling Unit permit. The city's DADU permit page lists every required document. Required with every application: a geotechnical report, a full plan set, an erosion control report, a traffic concurrency certificate, a utility service application, a water meter sizing worksheet and a state energy code worksheet. A critical areas study, structural calculations and health district approval are added when your lot calls for them.
• An ADU inside your existing house, like a basement conversion, or an addition that becomes an ADU, uses the Accessory Dwelling Unit permit. It is a lighter review.

Every ADU also gets a Level 1 land use review, which is the city's simplest review level, handled alongside the building permit. The ADU needs its own street address, and a four foot wide walkway has to run from the street or driveway to the ADU's front door.

The Self Certified DADU Program

Issaquah offers something most cities do not. If your detached ADU is not in a geologically hazardous area, a licensed architect can sign an affidavit certifying that the plans meet state and city codes, and the city streamlines its review. The architect has to carry at least $2 million in liability insurance and accepts random audits. The affidavit is uploaded with your application. For a flat lot, this is the fastest route to a permit in Issaquah.

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. Issaquah 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, two things in Issaquah push you toward licensed help. The self certified program only works with an architect's signature. And every detached ADU permit requires a geotechnical report, which must come from a licensed engineer. Structural calculations are required when the city asks for them, which is common on sloped lots and for anything unusual like a unit over a garage.

Whoever draws the plans, the drawings have to show property lines, setbacks, hard surface totals and every significant tree correctly. Drawings that miss these come back with comments, and every round of comments adds weeks.

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. Issaquah sits in a valley between Tiger, Squak and Cougar Mountains, with creeks running through most neighborhoods and old coal mines under some of them, so these come up more often here than in flatter cities. The sooner you know which ones apply to your lot, the fewer surprises you get later.

Slopes and Soil: The Geotechnical Report Is Not Optional

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 most cities you only need one on a sloped lot. In Issaquah, the DADU submittal checklist requires one for every new detached ADU, flat lot or not. Plan on it from day one. The city's soils report requirements spell out what it has to cover.

Beyond that baseline, the critical areas code in IMC 18.802.130 adds buffers when your lot has a mapped hazard. Here is what the city looks for:

• Steep slopes. Ground that rises 40 percent or more over at least 20 feet of vertical change. The city puts a 50 foot buffer at the top, bottom and sides of the slope. An engineer's study can shrink that to 10 feet, but an occupied building still has to stay 25 feet from the top or toe of the slope.
• Landslide hazard. Ground that slopes 15 percent or more with at least 15 feet of rise, plus a sign of past trouble like clay layers, seeps, hummocky ground or old slide debris. Same 50 foot buffer on all sides. An engineer's study can reduce it.
• Earthquake soil. Loose, wet soils along Issaquah Creek and the valley floor that can liquefy in a quake. Building is allowed, but the foundation has to be engineered for it.
• Old coal mines. Parts of Issaquah, especially around Squak Mountain and the Newcastle side, sit over abandoned mines. A severe hazard zone gets a 15 foot buffer and no buildings until it is remediated. A moderate zone needs an engineered foundation.

Any one of these also takes your project out of the self certified program. Issaquah does not publish how long a geotechnical report takes or a list of approved firms. Get schedules from local firms before you commit to a start date.

Streams and Wetlands

Issaquah Creek, Tibbetts Creek, North Fork and East Fork run through the middle of town, and dozens of smaller streams feed them. Each one comes with a buffer, which is a no build zone on each side. Stream buffers run 50 feet for a small seasonal stream, 75 feet for a year round stream without fish, and 150 feet for a stream with fish or a shoreline stream. Then the city adds a 15 foot building setback beyond the buffer. Wetland buffers run from about 40 to 300 feet depending on the wetland's category. The widths are in IMC 18.802.330.

A creek at the back of your lot that you have never paid attention to can take a big bite out of where an ADU fits. If your lot is within 200 feet of Lake Sammamish, the Shoreline Master Program applies on top of everything else. Check the city's critical areas map early.

Hard Surface: The Math That Decides How Big You Can Go

Issaquah caps impervious surface, not just building footprint. That includes your house, garage, driveway, walkways, patios, decks and the new ADU. For a detached cottage on a typical SF-S lot, this is usually the rule that sets the size. Here is how the math works.

In that example, a 1,000 square foot single story cottage does not fit. A two story design with a 500 square foot footprint does, with room for the required four foot walkway. A basement or garage conversion adds no new hard surface at all. Permeable pavers do not help here, because Issaquah does not count them toward your pervious area.

Two more things cut into the number. Critical areas and their buffers come out of your lot before the percentage is applied, so a creek buffer costs you twice. And county records are often fuzzy about your existing footprint. Records will not settle it. A real site plan or survey will.

Trees

Issaquah protects trees harder than most cities, and the rules got tighter again in 2025. A significant tree is any tree 6 inches across or more, measured 4.5 feet above the ground. For alder and cottonwood it is 8 inches. That is not a big tree. A landmark tree is 30 inches or more.

When you build a new detached ADU, two tree rules apply at once. You have to keep at least 35 percent of the total trunk diameter of all significant trees on the buildable part of your lot. And your lot has to meet a canopy target: 35 percent for lots of 5,000 to 9,999 square feet, 45 percent for lots of 10,000 to 14,999, and 60 percent for lots over 15,000. If you fall short, you plant replacement trees or pay into the city tree fund. Removing a significant tree needs a tree permit. The rules are in IMC 18.812.070.

In practice, 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. One useful break: if saving trees would make it impossible to fit the ADU, the city can waive parking entirely, which it already does not require.

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 and address. Issaquah requires its own address for the ADU and a four foot wide walkway from the driveway or street to its front door. A walkout or outdoor stairway still has to be checked against your foundation.

Fire Sprinklers and Fire Access

Issaquah requires sprinklers in new buildings over 5,000 square feet, so a 1,000 square foot ADU does not trigger that rule on its own. Two things can still bring sprinklers in. If the fire truck route to your lot is steeper than 12 percent, the fire marshal can require them. And if your water line cannot deliver enough flow to the new unit, sprinklers or a bigger line may be the fix. The water meter sizing worksheet in the application is where this gets sorted out.

Stormwater, Grading and Retaining Walls

Stormwater is rain that runs off roofs and paving instead of soaking into the ground. Issaquah follows the King County stormwater manual with its own additions. Once a project creates 2,000 square feet or more of new hard surface, drainage review kicks in and you need a plan to manage that runoff on your lot. Every detached ADU needs an erosion control report either way, and clearing on a mapped erosion hazard area is only allowed between May 1 and October 1.

A retaining wall over 4 feet tall needs its own permit and engineering. Many ADUs add somewhere around 500 to 1,500 square feet of new hard surface, which stays under the drainage line. A bigger unit with a new driveway can cross it. A real site plan with the driveway and walkway drawn in is how you know for sure.

Other Items to Confirm

• Water and sewer. The code allows an ADU to share the main house's water connection and meter as long as the fixture count and fire flow work. The city checks this through the utility service application and meter sizing worksheet. If you need a bigger meter or a new sewer connection, connection charges apply. Issaquah does not publish a simple ADU figure, so ask the utility that serves your lot before you set a budget. The city's site work and utility fee schedule has the current rates.
• Homeowner associations. Issaquah Highlands, Talus and many Klahanie and Sammamish Plateau neighborhoods have covenants. State law says covenants written before July 2023 can still block an ADU. Read yours before you spend money on drawings.
• Septic. A few lots on the city's edges are still on septic. An ADU on septic needs King County health department approval, which is one of the listed items on the DADU checklist.

Timeline

How Long Does an Issaquah ADU Permit Take?

Issaquah publishes target review times in weeks. They are targets, not promises, and the city says scope and complexity change them. 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 target review times handout, updated April 2026.

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

1. Jurisdiction check. Confirm your parcel is inside city limits and find your zone. This takes ten minutes and saves weeks.
2. Lot review. Your property lines, hard surfaces, slopes, streams and significant trees get measured on the ground, not pulled from records.
3. Design. Unit sizes and locations get tested against the setbacks, the trees, the hard surface cap and your yard. You should see options before anything is drawn for the city.
4. Reports. A geotechnical report is required for every detached ADU. A critical areas study and structural calculations are added on sloped or hazard mapped lots. Get real schedules from local firms before you commit to a date.
5. Permit submittal. The application goes in online through MyBuildingPermit.com with the plan set, reports and worksheets. If the lot is clear of hazards, an architect's self certification affidavit goes in with it.
6. City review. The city reviews the plans and sends comments. The design team answers them and resubmits as needed.
7. Permit issued. Once the city approves, construction can start.

The biggest swing factors in Issaquah are hazard mapping and trees. A flat lot away from creeks and mines can use the self certified program and move fast. A sloped lot near a stream waits on a geotechnical report and a critical areas study before the design can be locked. Settling those early is the best way to keep your timeline from slipping.