Sammamish ADU Permit Requirements, in Plain English

The short answer

Can I Build an ADU in Sammamish?

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. Sammamish 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 with its own kitchen, bathroom and entrance, it is an ADU, and the rules on this page apply.

Sammamish rewrote its ADU rules on January 1, 2025 to match state law, and has adjusted them twice since. The old limits are gone. You no longer have to live on the property. You can have two ADUs instead of one. A detached unit can now be 24 feet tall instead of 18, and the old rule that it match your house's siding, roof and windows was dropped. The city's own one page DADU handout still shows the 2023 rules, so if you read it, know that it is out of date.

Today, on any lot with a single family home, you can build up to two ADUs in any mix of attached and detached. Each one can be up to 1,000 square feet. Most lots with a two car garage and a driveway need no new parking. This page walks through the current rules and the things that actually shape a project in Sammamish.

First, Make Sure Sammamish Is Your Permit City

Your mailing address and your permit city are not always the same. Plenty of homes with a Sammamish address sit in unincorporated King County, especially east of the city toward the Snoqualmie Valley and along parts of the lakeshore. King County has its own ADU rules, and on rural zoned land a detached ADU needs a much bigger lot. Before you plan anything, look up your parcel on the Sammamish Property Tool or the King County Parcel Viewer. If the city holds your parcel, the rules on this page apply to you.

Know Your Zone

Sammamish has six residential zones, named for how many homes fit on an acre: R-1, R-4, R-6, R-8, R-12 and R-18. All six allow up to two ADUs. Most single family neighborhoods in Sammamish are R-4 or R-6. The large lot areas around Beaver Lake and Pine Lake and in the north end are often R-1. Your zone decides your front setback and a few other details below, so check it on the Property Tool before you plan.

Confirmed rules

Sammamish ADU Rules: What Is Confirmed

Zoning and How Many Units You Can Build

Sammamish allows up to two ADUs on any lot with a single family home in any residential zone. You can mix them any way you want. Two attached, two detached in one or two buildings, or one of each. You can also convert an existing structure, like a detached garage, into an ADU. There is no minimum lot size for an ADU.

One thing to know. Each ADU counts as one dwelling unit when the city calculates density. On a standard lot that never matters. On an unusually small lot in a low density zone, ask the city to confirm before you plan two units.

Size Limits

Each ADU can be up to 1,000 square feet. That is enough for a two bedroom, two bath home. There is one exception. If an ADU is built entirely inside your existing house, like a basement or an attic conversion, it can be up to half the floor area of the house or 1,000 square feet, whichever is more. On a 3,000 square foot house, that means a basement ADU could be 1,500 square feet.

Sammamish also caps how much of your lot can be built on through a floor area ratio and a hardscape limit. An ADU built under the ADU rules is exempt from both. That is a real advantage on a lot where the house is already near the cap.

Height Limits

A stand alone detached ADU can be up to 24 feet tall, measured from the average existing grade of the ground to the highest point of the roof. That leaves room for two stories. If you combine the ADU with a garage, pool house or covered patio into one building, the height limit rises to 35 feet, but the building has to use the larger setbacks in the next section. An attached ADU follows the same height limit as your main house.

On a sloped lot, the city measures height in segments so a walkout design is not penalized for the downhill side. Your designer will know how to work that math.

Setbacks

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

A stand alone detached ADU gets the small setbacks. It can sit 5 feet from each side line and 5 feet from the rear line in every residential zone. The front setback follows the accessory structure rule for your zone.

Any front setback along an arterial street has to be at least 30 feet. If your lot's side line touches a lot zoned R-8, R-12 or R-18, some side setbacks can drop to 5 feet even for the main house.

A detached ADU that is combined with another structure, like an ADU over a detached garage, uses the regular accessory structure setbacks: 10 feet from the side and rear in R-1 through R-8, and 5 feet in R-12 and R-18. Same front setback as above.

An attached ADU follows the same setbacks as your main house. In R-4 and R-6 those scale with the size of the house, from 5 feet on the side and 12 to 15 feet in the rear for a house under 2,500 square feet, up to 10 feet on the side and 20 to 25 feet in the rear for a house over 4,000 square feet. R-1 needs 25 feet on the side and 30 in the rear.

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

Owner Occupancy

Sammamish does not require you to live on the property. The city removed that rule on January 1, 2025, and state law now bans it. You do not have to live on site to build or rent an ADU. If you still see a Sammamish form that asks for an owner occupancy notice recorded at King County, that is the old rule.

Parking

Most Sammamish ADUs need no new parking space. If your lot already has four or more spaces, counting the garage and the driveway, no additional parking is required. A two car garage with a two car driveway in front of it clears that bar. If your lot is within a quarter mile of a transit stop, no parking is required either way. Otherwise the city can require parking, and state law caps what it can ask for at one space per ADU on a lot under 6,000 square feet and two spaces on a larger lot.

If you do add parking in R-1 through R-8, the driveway has to be at least 12 feet wide, no more than 30 percent of the lot width, and the parking has to sit at least 20 feet back from the street line.

Renting It Out

You can rent your ADU, and you do not have to live next door. The ADU code sets no minimum rental length, and we found no citywide short term rental rule that applies to ADUs. That does not mean there is none, so ask the Permit Center before you list a unit for nightly stays. Long term rental is clearly allowed.

Fees

This is where Sammamish changed the most in 2026, and where old information will throw you off.

On July 1, 2026, the city replaced its flat impact fees with fees charged by the square foot. An impact fee is a one time charge that helps the city pay for parks and roads as the city grows. The parks fee is $3.13 per square foot and the transportation fee is $7.44 per square foot, for $10.57 combined. Both have a minimum charge based on 783 square feet and a maximum based on 3,500 square feet. The city's impact fee page has the current rates.

State law says an ADU's impact fee cannot be more than half of what a new house pays. Under the square foot system, a 1,000 square foot ADU already lands well under half of a typical new house, so the cap rarely changes the number. Ask the city how it applies the cap to your project anyway, especially for a small unit paying the minimum. The city also allows impact fees to be deferred until the unit is finished, against a lien on the property. Ask about that if you want to hold cash during construction.

School impact fees depend on which school district your lot sits in, and they are not charged by the square foot. The city's 2026 fee schedule lists a per unit fee for the Lake Washington, Issaquah and Snoqualmie Valley districts, plus a $266 administration fee. The published rates differ a lot by district, and the schedule does not say whether an ADU pays the full house rate, half, or nothing. Confirm your district and the ADU rate with the Permit Center before you budget. The north half of Sammamish is mostly Lake Washington, the south half mostly Issaquah.

Building permit and plan review fees are set by the construction value. For a project valued at $100,000 to $500,000, the permit fee is $1,278.51 plus $8.65 for every $1,000 over $100,000. Plan review adds 65 percent of that, and a 15 percent technology fee sits on top. On a $400,000 ADU that works out to roughly $7,400 before mechanical, plumbing and energy code fees. Add a $2,248 engineering review fee if your lot has a critical area, and a one time surface water charge of $3,798 that can apply to a new building.

One fee you do not pay. Sammamish charges a $34.45 per square foot affordable housing fee on big new houses. Since August 2026 that fee only applies above 1,750 square feet, so a 1,000 square foot ADU is exempt.

Water and sewer connection charges are set by your water district, not the city. Most of Sammamish is served by the Sammamish Plateau Water and Sewer District. The northeast corner is served by the Northeast Sammamish Sewer and Water District. Ask your district directly what a second unit costs to connect.

How You Apply

Start with the city's free Project Guidance request. You describe the project, and within about a week the city sends back the setbacks, critical area designations and next steps for your specific parcel. It costs nothing and it catches surprises early.

You then apply online through MyBuildingPermit.com. An ADU needs only a building permit. There is no hearing, no neighbor notice, and no document to record at the county. Which checklist you follow depends on what you are building.

• A new detached ADU, or a new addition that becomes an ADU, follows the new single family residence checklist. Expect a site plan, floor plans, elevations, structural drawings and calculations, a drainage plan, a tree plan, and a critical areas worksheet. Sammamish treats a new ADU like any other new home for permitting.
• An ADU created inside your existing house, like a basement conversion, follows the residential remodel checklist. It is a lighter review.

The city's Applications and Forms page has every checklist. The Permit Center is at 801 228th Avenue SE, with walk in hours Monday and Friday mornings and Wednesday afternoons, at (425) 295-0531 or permittech@sammamish.us.

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. Sammamish 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, a Sammamish submittal is not light. Your site plan has to show property lines, setbacks, every significant tree, and any critical area or buffer correctly. Your drawings 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. Structural calculations usually need an engineer's stamp. Retaining walls over 4 feet and unusual structural work, like cutting new openings into a foundation, need one too. If your lot is flat and the design is simple, you may never need one beyond structural. 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. Sammamish was built on a forested plateau with lakes, ravines and a lot of older septic systems, so these come up often. The sooner you know which ones apply to your lot, the fewer surprises you get later.

Trees

This is the complication Sammamish homeowners most often do not see coming. The city protects trees harder than most of its neighbors, and the rules on a new building are tied to how many you keep.

A significant tree is a conifer 8 inches across or more, or a deciduous tree 12 inches across or more, measured 4 feet above the ground. That is not a big tree on the plateau. When you build, the city requires you to keep a share of the significant trees on the lot outside of critical areas: 35 percent in R-4 and R-6, and 50 percent in R-1 and in the Pine Lake and Beaver Lake drainage areas in any zone. Removing a significant tree needs a permit, which costs $139, and cutting one without a permit carries a penalty of $1,500 per inch of trunk width. Outside of a construction project, the city also limits how many trees you can take out per year and per decade based on lot size. On a site where construction is planned, the city does not let you remove trees until the building permit is issued.

In practice, this means a stand of firs in your backyard can decide where the ADU goes, not the other way around. Walk your lot and count the conifers over 8 inches across before you fall in love with a location. The city's tree management page covers the basics, and the tree rules are in SMC 21.03.060.

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 Sammamish, 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. The city rewrote these rules on January 1, 2025, and the current version is in SMC 21.03.020.

Here is what the city looks for:

• Landslide hazard. Ground that drops 40 percent or more with at least 10 feet of rise, or ground with signs of past movement. The city puts a 50 foot buffer at the top and the bottom of the slope. Nothing gets built inside it without an engineer's study.
• Erosion hazard. Steep ground with loose soil, common along the ravines that run down to Lake Sammamish. Building here means an erosion control plan and often a soil report.
• Earthquake soil. Soil rated moderate to high for liquefaction, which means loose, wet soil can act like liquid during an earthquake. Rare on the plateau, more common near the lake.

Those buffers can shrink. If a licensed engineer or geologist studies the site and shows the project is safe, the city can reduce or remove the buffer. That study is what you are paying for. A critical area on your lot also adds a $2,248 engineering review fee, can require a deposit of up to $5,000 for an outside expert to review your report, and takes away some of the state law protections that cap ADU setbacks and fees. The city does not publish how long a soil report takes. Get schedules from local firms before you commit to a start date.

Streams, Wetlands and Lakeshore

Sammamish has more water on it than most people realize. Small streams drain the plateau on every side, and many back yards hold a wetland that only shows itself in winter. Each one comes with a buffer, which is a no build zone around it. A stream or wetland you have never noticed at the back of your lot can take a big bite out of where an ADU fits. The Property Tool's sensitive areas layer is a first look. A real wetland delineation is how you know.

Lots on the lakes follow the shoreline code on top of everything else. New buildings need 50 feet from the ordinary high water mark on Lake Sammamish, and 45 feet on Pine Lake and Beaver Lake. On Lake Sammamish that setback can be reduced, but never below 20 feet, and only through a shoreline review. Plan on extra time if your lot touches the water.

Septic Systems

This one is specific to Sammamish and it matters. A large share of the city's older lots, especially around Pine Lake, Beaver Lake and the south end, are on septic systems instead of sewer. An ADU adds bedrooms, and a septic system is sized by bedrooms. Adding a unit to a septic lot means King County Public Health has to approve the system for the new load, which often means a larger drainfield, a new reserve area, or a full system replacement. On a small lot, there may not be room for it.

Before you plan anything on a septic lot, pull your septic record from King County Public Health and find out whether sewer is available at your street. If it is, connecting to sewer may be the better move. Your water district can tell you what a connection costs.

Fire Sprinklers and Fire Access

Sammamish has required fire sprinklers in very large homes for years, with the trigger set at 5,000 square feet. A 1,000 square foot ADU does not hit that on its own. But Eastside Fire and Rescue reviews every new dwelling, and sprinklers can be required for other reasons: a long or narrow driveway a fire truck cannot reach down, or a lot too far from a fire hydrant. Deep lots with a cottage tucked far behind the house are where this shows up. Ask the city at Project Guidance whether your site has a fire access issue. It is far cheaper to know before the design is drawn.

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. A walkout or outdoor stairway has to be checked against your foundation and your setbacks.

Stormwater, Grading and the Wet Season

Stormwater is rain that runs off roofs and paving instead of soaking into the ground. Sammamish follows the King County stormwater manual with its own additions. Once a project adds or replaces about 2,000 square feet of hard surface, you need a plan to manage that runoff on your lot, and more rules kick in at larger sizes. A new ADU with a driveway and walkways can cross that line even when the building itself is small. A real site plan with every hard surface drawn in is how you know for sure.

Clearing and grading needs its own permit once you move more than a small amount of dirt, and the city restricts clearing and grading in the wet season. Plan on earthwork between May 1 and September 30. Starting in October can mean sitting on a permit until spring, or paying for extra erosion control to work through the rain.

Other Items to Confirm

• Pre approved plans. As of October 2026, Sammamish does not offer pre approved ADU plans. Budget for a custom design.
• Building code edition. Sammamish is on the 2021 state codes. The 2024 codes are scheduled to take effect May 3, 2027. Which edition applies depends on when you apply, so a project that submits in early 2027 should check which rules it will be reviewed under.
• Staffing. In the fall of 2026 the city warned that staff transitions may slow permit service. Build slack into your schedule.

Timeline

How Long Does a Sammamish ADU Permit Take?

Sammamish does not publish review timelines for building permits. Two things it does commit to: Project Guidance requests are answered within about a week, and tree removal applications are screened within two business days and reviewed within five. Everything else depends on the queue and on how clean your submittal is.

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

1. Lot review. Your property lines, hard surfaces, slopes, streams, significant trees and septic or sewer status get measured on the ground, not pulled from records.
2. Project Guidance. The city's free review confirms your zone, setbacks and critical areas in about a week. Do this before design, not after.
3. 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.
4. Reports, if required. A tree plan is required for every new building. A geotechnical report is required on sloped or hazard mapped lots. A wetland delineation is required if the sensitive areas map flags your lot. A septic lot needs King County Public Health sign off. None of these have a published turnaround, so get real schedules before you commit to a date.
5. Permit submittal. The application goes in online through MyBuildingPermit.com with the site plan, floor plans, structural drawings, drainage plan, tree plan and critical areas worksheet.
6. City review. The city reviews the plans and sends comments. The design team answers them and resubmits as needed. A third round of review costs $182 an hour, so getting it right in two rounds pays.
7. Permit issued. Once the city approves, construction can start. On a wooded lot, that is also when the trees can come down.

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