Redmond ADU Permit Requirements, in Plain English

The short answer

Can I Build an ADU in Redmond?

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, it is an ADU, and the rules on this page apply.

Redmond rewrote its ADU rules in June 2025 to match state law. The old limits are gone. You no longer have to live on the property. The old one unit cap is gone. The old parking space is gone. The old deed restriction you had to record with the county is gone. If you read about Redmond ADUs more than a year ago, most of what you read is out of date, and some older copies of the city code still show the old rules.

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. Every new ADU in Redmond needs fire sprinklers, and that is the rule most homeowners do not see coming. This page walks through the current rules and the things that actually shape a project.

First, Make Sure Redmond Is Your Permit City

This matters more in Redmond than almost anywhere else on the Eastside. Thousands of homes with a Redmond mailing address sit in unincorporated King County, not in the city. Union Hill, Novelty Hill, Redmond Ridge, Ames Lake, and parts of English Hill and the Bear Creek valley are all outside city limits. Those lots follow King County's rules, not Redmond's.

The difference is not small. On county land zoned rural, a detached ADU needs a lot of at least 3.75 acres. A one acre lot with a Redmond address and county zoning cannot add a backyard cottage at all. It can only add an attached unit inside or onto the house. Before you plan anything, look up your parcel on the King County Parcel Viewer. The jurisdiction line tells you who holds your permit. If it says Redmond, the rules on this page apply to you. If it says King County, they do not.

Your Zone Has a New Name

In November 2024, Redmond replaced its old single family zones. If your plat or an old permit says R-4, R-5 or R-6, your lot is now in the Neighborhood Residential zone, which the city calls NR. Most Redmond neighborhoods are NR, and this page uses NR numbers throughout. If your lot is in a mixed use or downtown zone, ADUs are still allowed, but the setbacks and heights on this page may not match yours.

Confirmed rules

Redmond ADU Rules: What Is Confirmed

Zoning and How Many Units You Can Build

Redmond allows up to two ADUs on any residential lot. 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. If your lot is big enough for the house that is already on it, it is big enough for an ADU.

The city also allows tiny homes on wheels as a kind of ADU, up to 400 square feet, as long as the trailer base is screened from the street with landscaping. Each tiny home counts as one of your two units.

Size Limits

Each ADU can be up to 1,000 square feet. That is enough for a two bedroom, two bath home. Redmond does not add a bonus for garages or storage the way some cities do, so plan on the whole structure counting.

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 your lot is over 10,000 square feet and the unit is set aside as affordable housing or gives the city some other public benefit.

There is a second ceiling that catches people on big houses. Redmond caps the total floor area of every building on the lot together. With two homes on the lot, the house and the ADU combined cannot pass 6,000 square feet, and below grade floors count. A 5,500 square foot house leaves room for only a 500 square foot ADU, no matter how big the yard is.

Height Limits

A detached ADU follows the same height limit as a house in your zone. In NR that is 38 feet, which is plenty for two stories. This is more generous than most Eastside cities and more than the 22 foot limit Redmond puts on sheds and garages. An attached ADU follows the height limit of your main house.

Setbacks

A setback is the minimum distance between a building and your property line. Redmond's are tight, which is good news for fitting a cottage into a small backyard.

There are two more breaks. If a detached ADU sits next to an alley or a public right of way, it can be built right on that lot line with no setback at all. And a detached ADU does not need any set distance from the main house unless the building or fire code calls for one.

One limit to know: Redmond keeps accessory buildings out of the front yard. A backyard cottage goes in the back or the side, not between the house and the street.

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

Owner Occupancy

Redmond does not require you to live on the property. The city dropped that rule in 2025, and state law now bans it. The old requirement to record a deed restriction with King County is gone too. You do not have to live on site to build or rent an ADU.

Parking

No new parking space is required for an ADU anywhere in Redmond, at any size. You can add one if you want to. The city does not make you.

Renting It Out

You can rent your ADU, and you do not have to live next door. Long term rentals follow the same landlord rules as any other home. If you want to rent it for stays under 30 nights, Redmond started requiring a short term rental business license and a safety checklist on January 1, 2026. Get that sorted before you list it.

Fire Sprinklers

This is the Redmond rule that surprises people. The fire department requires residential fire sprinklers in every new ADU, attached or detached, at any size. There is no small unit exception. A detached cottage needs its own sprinkler system and a separate fire permit. An attached ADU can also pull the main house into the sprinkler requirement if there is no fire rated separation between the two units. Sprinklers add real cost and can affect your water service size, so they belong in the budget from day one. The rule is in the fire department's residential sprinkler ruling.

Fees

Impact fees are one time charges that help the city pay for roads, parks, fire service and schools as the city grows. Redmond's 2026 schedule does not have a line that says ADU. The city files an ADU under middle housing for the transportation fee, which is $5,757 in 2026. Parks and school fees are published per housing unit with no ADU row. State law caps the total impact fees on an ADU at half of what a new single family house would pay, and a new house in Redmond pays over $10,000 for transportation alone. Ask the permit desk for a written ADU fee number before you budget. The fees adjust every January 1.

Building permit fees are based on the construction value of the project. For a detached ADU valued around $300,000, the building permit, plan review, planning review and fire review together run roughly $8,000 to $9,000 before impact fees and utility charges. The city's 2026 impact fee ordinance and its development fee schedules have the current figures.

Water and sewer connection charges depend on your situation. Redmond allows an ADU to share the house's water meter as long as both units stay under the same owner. Otherwise a separate meter is required. Sprinkler service can be combined with the domestic line or run separately. Ask Redmond Utilities what your lot needs before you set your budget.

How You Apply

You apply online through Redmond's Plan Review Online system. Paper plans are not accepted. First time applicants have to book an intake appointment with a permit technician. An ADU uses a standard residential building permit. There is no separate ADU land use permit, no public notice, and no hearing.

• A new detached ADU, or a new addition that becomes an ADU, uses the residential building permit for new construction. Required documents include a site plan, floor plans, elevations, structural calculations, a tree preservation plan with an arborist report, a stormwater site plan, and a separate fire sprinkler permit.
• An ADU created inside your existing house, like a basement conversion, uses the residential alteration permit. It is a lighter review, but the sprinkler rule still applies to the new unit.

The city's permit technicians can be reached at permits@redmond.gov or 425 556 2473, and a planner on call answers zoning questions at 425 556 2494.

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. Redmond 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, Redmond asks for a full package. A new ADU permit needs structural calculations, a tree preservation plan with an arborist report, a stormwater site plan and a sprinkler design with the application. Your site plan has to show property lines, setbacks, lot coverage, total floor area on the lot and every significant tree correctly. 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. The sprinkler design comes from a licensed sprinkler contractor. 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 an engineer beyond the sprinkler designer. If your lot is sloped or near a stream, plan on it.

Watch for these

What Can Complicate Your Project

None of these usually kill a project inside the city. They change where the unit goes, what it costs, or how long it takes. Redmond's older neighborhoods have big trees and small lots, and the city sits on top of streams, wetlands and the aquifer that supplies its drinking water. The sooner you know which of these 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 Redmond, 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. Redmond's rules for slopes and soil are in RZC 21.64.060.

Here is what the city looks for:

• Landslide hazard. Any slope of 40 percent or more with at least 10 feet of rise. Also any slope over 15 percent with springs or seeping groundwater, and any area that has slid before. The city puts a 50 foot buffer around the top, bottom and sides of the slope. Nothing gets built inside that buffer, and nothing gets built on the slope itself.
• Erosion hazard. Certain soil types on slopes of 15 percent or more that the federal soil survey rates as highly erodible. There is no automatic buffer, but the city requires a study before you disturb the ground.
• Earthquake soil. Ground at risk of liquefaction, settlement or faulting in an earthquake. Liquefaction means loose, wet soil can act like liquid during shaking. The Sammamish River valley has some of this.

The 50 foot landslide buffer can shrink. If a licensed engineer or geologist studies the site and shows the risk is low, the city can cut the buffer down to 15 feet. The city can also make it bigger if it thinks the slope is worse than the map shows.

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. Education Hill, the slopes above the Sammamish River, and the hillsides along Bear Creek all have stretches of it. If your lot does, plan for a soil report. Redmond does not publish how long one takes. Get schedules from local firms before you commit to a start date.

Streams, Wetlands and the Aquifer

Redmond has more water running through it than most Eastside cities. The Sammamish River, Bear Creek, Evans Creek and a dozen smaller streams each come with a buffer, which is a no build zone on each side. Stream buffers run from 50 to 200 feet depending on the stream type. Wetland buffers run from 50 to 300 feet depending on the wetland category. A seasonal stream you have never noticed at the back of your lot can take a big bite out of where an ADU fits.

Redmond also gets its drinking water from wells under the city. Much of the city sits in a Critical Aquifer Recharge Area, which limits what can go into the ground. For a house this mostly affects stormwater design, since the city will want runoff handled in a way that does not carry pollution into the aquifer.

One more thing. State law gives ADUs a lot of protection from local rules, but that protection stops at critical areas and their buffers. If your lot has a stream, a wetland or a mapped slope, the city's full critical areas rules apply, no exceptions.

Lot Coverage and Total Floor Area: The Math That Decides How Big You Can Go

Lot coverage is how much of your lot is covered by buildings. Redmond caps it at 50 percent of the lot. That count includes your house, your garage, any sheds, and the new ADU. The city also caps hard surface, which adds driveways, walkways and patios to the count, at 70 percent. And on top of both, the city caps the total floor area of every building on the lot. Here is how the math works.

In that example, both a single story 1,000 square foot cottage and a two story design fit, and the floor area cap is nowhere close. On a 5,000 square foot lot with a 2,000 square foot house footprint, the footprint math gets tight fast, and a two story cottage with a 500 square foot footprint is the way to fit a full 1,000 square feet. A basement conversion adds no new footprint at all.

Two more checks. Redmond requires 20 percent of the lot to stay open space. And the city caps the footprint of accessory buildings at 60 percent of the main house's ground floor living area, not counting the garage. A small rambler with a big backyard can run into that one before it runs into lot coverage. Confirm with the city whether that cap applies to your detached ADU.

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

Redmond protects trees hard, and the city's older neighborhoods are full of them. A significant tree is any tree 6 inches across or more, measured 4.5 feet above the ground. That is not a big tree. A landmark tree is over 30 inches across. Removing any significant tree needs a permit. Removing a landmark tree needs a special exception request.

For new construction, the rules get tighter. Every construction permit application has to include a tree preservation plan and an arborist report. You have to keep at least 35 percent of the significant trees on the lot. Each significant tree you remove gets replaced one for one, and a landmark tree gets replaced three for one, mostly with native species. The city's tree removal page covers the basics.

In practice, this means a big tree in the middle of your backyard can decide where the ADU goes, not the other way around. The root zone matters too, not just the trunk. Walk your lot and count anything over 6 inches across before you fall in love with a location.

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.

In Redmond this matters twice. The new ADU needs sprinklers no matter what. But if you skip the fire rated separation, the city can require sprinklers in the whole house too, not just the new unit. A proper separation keeps the sprinkler work contained to the ADU.

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 building code wants 7 feet of ceiling height in living space. Redmond's building official can waive that for a basement built under an older code, which is a real break for mid century homes. 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.
• Sprinklers. A converted basement is a new ADU, so it needs sprinklers, and the water service to the house may need to be upsized to feed them.

Stormwater, Grading and Retaining Walls

Stormwater is rain that runs off roofs and paving instead of soaking into the ground. Redmond follows the state stormwater manual with its own local notebook. The thresholds work in tiers. Once a project creates or replaces 2,000 square feet or more of hard surface, you need a stormwater site plan and on site measures like rain gardens or dispersion trenches. At 5,000 square feet or more, you need engineered flow control, which usually means a buried detention tank. Every new ADU gets a stormwater review either way, and the aquifer rules push the city toward infiltration where the soil allows it.

Many ADUs add somewhere around 500 to 1,700 square feet of new hard surface, which stays under the 2,000 square foot 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.

A retaining wall over 4 feet tall, measured from the bottom of the footing, needs its own permit and an engineer's design. Moving a lot of dirt or clearing a large area triggers a separate clearing and grading permit. On a sloped Redmond lot, these add up quickly.

Other Items to Confirm

• Water and sewer. An ADU can share the house's water meter if both units stay under the same owner. Sprinklers may require a larger service line. Some Redmond addresses are served by a water or sewer district rather than the city, and districts have their own connection fees. Ask Redmond Utilities what your lot needs and what it costs.
• Selling the ADU separately. Redmond removed its ban on separate ownership in 2025, and the city allows unit lot subdivisions for attached and detached units. It is a separate land use process with its own fees and rules. Ask the planner on call before you count on it.
• Pre approved plans. Redmond does not offer pre approved ADU designs as of October 2026. Every project goes through full plan review.

Timeline

How Long Does a Redmond ADU Permit Take?

Redmond does not publish a review target for ADU permits the way Bellevue does. What you have instead is state law. For a permit with no public notice, which includes every ADU building permit, the city has to issue a decision within 65 days of calling your application complete. If it misses that deadline, you get part of your permit fee back. The clock stops every time the city sends you comments and restarts when you resubmit.

Most projects go through two or three rounds of comments. A clean, complete first submittal is the single best way to keep that number low. Redmond's own review timeframes are in RZC 21.76.040.

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. In Redmond, this is also where you confirm you are inside city limits.
2. Design. Unit sizes and locations get tested against the setbacks, the trees, the slope, the floor area cap 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. A sprinkler design is required for every ADU. None of these 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 with the site plan, floor plans, structural calculations, tree preservation plan, stormwater site plan and sprinkler permit.
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 permit is good for 365 days, and each passed inspection extends it another 365.

The biggest swing factors in Redmond are trees, sprinklers and water. A flat lot with no big trees and a water line big enough for sprinklers can move fast. A wooded lot on a slope that needs a water service upgrade 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.