Bothell ADU Permit Requirements, in Plain English

The short answer

Can I Build an ADU in Bothell?

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. Bothell 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.

Bothell rewrote its ADU rules in 2024 and 2025. The old limits are gone. You no longer have to live on the property. The old 800 square foot cap is gone. The one stall parking requirement is gone. The recorded covenant is gone. If you read about Bothell ADUs more than a year or two ago, most of what you read is out of date.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. This page walks through the current rules and the things that actually shape a project.

First, Make Sure Bothell Is Your Permit City

This matters more in Bothell than almost anywhere on the Eastside. The city sits in two counties, King and Snohomish, and a lot of homes with a Bothell mailing address are not in the city at all. North of the city limits, many "Bothell" homes sit in unincorporated Snohomish County. South and east, some sit in unincorporated King County or in a neighboring city. Each of those has its own ADU rules, and on rural zoned county land a detached ADU can need acres you may not have.

Before you plan anything, look up your parcel on the city's Maps and GIS page. If the city holds your parcel, the rules on this page apply to you, whichever county you are in. The county still matters for two things: septic sign off comes from King County Public Health or the Snohomish Health District, and your water and sewer certificate comes from whichever provider serves your lot (the City of Bothell, Alderwood, Northshore or Woodinville).

Your Zone Has a New Name

In 2025, Bothell replaced its old residential zone names, like R 9,600 and R 7,200, with a new set. Your plat, your old permits and most websites still use the old names. The new ones are R-C (Residential Conservation), R-L1 and R-L2 (Residential Low) and R-M1 through R-M4 (Residential Medium). All of them allow ADUs. Most single family neighborhoods in Bothell are now R-L1 or R-L2. Find your current zone on the city's zoning map and use that name, not the one on an old document.

Confirmed rules

Bothell ADU Rules: What Is Confirmed

Zoning and How Many Units You Can Build

Bothell allows up to two ADUs on any lot where a single family home 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 beyond what your zone already requires for the house.

Two exceptions. If your lot already holds middle housing, such as a duplex or townhomes, you get one ADU. And if your lot has a designated critical area or its buffer, such as a stream, a wetland or a steep slope, you get one ADU even if the critical area is nowhere near where you want to build. That second rule is Bothell's own, not the state's, so it is worth a call to the city if it affects you.

Size Limits

Each ADU can be up to 1,200 square feet. That is enough for a two bedroom, two bath home. A detached ADU does not count against the city's rule that accessory buildings cannot cover more than 5 percent of the lot, so the size cap and the hard surface cap are the only two numbers that limit a backyard cottage.

If you are converting space inside your existing house, the Community Development Director can approve a unit larger than 1,200 square feet, as long as the ADU is no more than 40 percent of the main home's gross floor area. That covers a big daylight basement without forcing you to wall part of it off.

Height Limits

A detached ADU can be up to 30 feet tall. That is more generous than Bellevue, Kirkland or Redmond, and it leaves easy room for two full stories. If you build the ADU on top of an existing garage or shed, the limit rises to 33 feet. An attached ADU follows the same height limit as your main house, which is 35 feet in the R-C, R-L and R-M1 and R-M2 zones.

Two story detached units do come with privacy rules. Within 10 feet of a property line, second floor windows with low sills need obscured glass, and second floor decks have to face away from the neighbors or carry a 6 foot screen. Plan the upstairs layout with that in mind.

Setbacks

A setback is the minimum distance between a building and your property line. Bothell keeps this simple. A detached ADU uses the same setbacks as the main house, and the side setback is 5 feet in every residential zone.

A garage door has to sit 20 feet back from the street in every zone, so a car can park in the driveway without hanging into the sidewalk. If your lot touches an alley, the alley setbacks in BMC 12.14.060 apply on that side instead, and they are shorter.

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

Bothell does not require you to live on the property. The old rule that the owner or a family member had to live in one unit for more than six months a year, and record a covenant against the title, is gone. State law now bans it. You do not have to live on site to build or rent an ADU.

Parking

No parking is required for an ADU anywhere in Bothell. The city used to require one extra paved stall per ADU. Ordinance 2461, adopted July 8, 2025, removed that rule and the minimum parking table that went with it. You can still build a space if you want one. Your site plan has to show the existing and proposed parking for both units either way, so the city can see what is there.

Renting It Out

You can rent your ADU, and you do not have to live next door. Bothell's ADU section has no rule on short term rentals. The city's general business license, rental and nuisance rules still apply, and so does anything in your plat or homeowner association covenants. If nightly rentals are part of your plan, ask the city directly before you count on them.

Fees

Bothell charges impact fees on a new ADU, but caps them at half of what a new house pays. An impact fee is a one time charge that helps the city pay for roads, parks and fire service as the city grows. In 2026 the transportation fee for an ADU is about $3,300, the fire fee is a few hundred dollars, and the park fee runs from roughly $800 to $2,000 depending on your zone. You pay the rate in effect when your permit is issued, not when you apply, and the transportation rate went up on May 1, 2026.

School impact fees are collected for the Northshore School District, and the city's fee form does not list a separate ADU rate. Ask the city which rate an ADU pays before you set your budget. That one question can swing the number by thousands of dollars.

Building permit and plan review fees are based on the value of the work. The plan check fee is 65 percent of the building permit fee and is due when you apply, along with a $231 planning review fee. At issuance you pay the building permit fee, plumbing and mechanical fees at 8 percent each for a new detached unit, an $834 site review and inspection fee, a stormwater facility charge, and a 5 percent technology fee on most of the above. For a typical detached ADU, plan on several thousand dollars in permit fees before impact fees.

Water and sewer connection charges are set by your provider, not the building department. If the City of Bothell serves your lot, start with a Water Sewer Availability application (Form 36). It takes up to two weeks to process and you need it before the city will accept your permit.

How You Apply

You apply online through MyBuildingPermit.com under Building, then Single Family Residential, then Accessory Dwelling Unit. Before the city accepts the application, you need an assigned address letter for the new unit (Form 54) and a water and sewer availability certificate from your provider.

• A new detached ADU, or a new addition that becomes an ADU, needs a site plan, full floor plans and elevations, framing and foundation plans, structural calculations for anything not built to the prescriptive tables, full Washington State Energy Code worksheets, a fire flow letter from your water provider, and a hard surface coverage table for the whole lot.
• An ADU created inside your existing house, like a basement conversion, needs floor plans showing the existing and proposed layout, ceiling heights, egress windows and alarm locations, plus the energy worksheet for a remodel. It is a lighter review.

The city's ADU permit checklist lists every required document. If you are on septic and adding square footage or bedrooms, you also need sign off from King County Public Health or the Snohomish Health District, depending on which county your lot is in.

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. Bothell 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, the drawings have to be right. Bothell asks for a site plan at a set scale with contours, easements, every structure, the hard surface table and the drainage layout. Floor plans have to show ceiling heights, egress window sizes and sill heights, and alarm locations. 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 are required for any beam, header or foundation that is not sized from the prescriptive code tables. An on site drainage dispersal system has to be designed and stamped by a geotechnical engineer. 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. Bothell is easier than most Eastside cities on trees and parking, and tougher than most on hard surface. The sooner you know which ones apply to your lot, the fewer surprises you get later.

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

This is the number that shapes more Bothell ADU projects than any other. Hard surface is everything on your lot that rain cannot soak through: roofs, driveways, walkways, patios and decks. Bothell caps it at 55 percent of the lot in R-L1, 60 percent in R-L2, 70 percent in R-M1, 75 percent in R-M2 and 35 percent in R-C. The city dropped its separate building coverage cap in 2025, so this one number now does all the work. 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 left for a walkway. A basement or garage conversion adds no new hard surface at all. Permeable paving, which lets rain soak through, can also buy you room, because the city's coverage table has a column for it.

County records are often fuzzy about your existing footprint. They may not show the driveway, the patio or the true outline of the house. Bothell requires a hard surface coverage table on your site plan, and the city checks it. Records will not settle it. A real site plan or survey will.

Streams, Wetlands and Slopes: Critical Areas

Critical areas are places the city treats as sensitive: streams, wetlands, steep slopes and unstable soil. Bothell has a lot of small streams feeding the Sammamish River and North Creek, and a lot of slopes along them. Each critical area comes with a buffer, which is a no build zone around it. Your site plan has to show any critical area on your lot or within 200 feet of it.

In Bothell a critical area costs you twice. First, the buffer takes away ground you cannot build on. Second, a lot with a designated critical area or buffer is limited to one ADU instead of two. A stream you have never noticed at the back of your lot can decide both how many units you get and where the one unit goes.

If your lot has a slope, plan for 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. The city's critical areas code in BMC Title 14 is what triggers one, and the city's site plan rules already require 5 foot contours once a slope passes 15 percent. 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. Bothell does not publish how long a report takes or a list of approved firms. Get schedules from local firms before you commit to a start date.

Trees

Here Bothell gives most homeowners a break. The city's tree retention code in BMC 12.18.030 exempts an existing single family home on a lot of 20,000 square feet or less. If that is you, and it is most Bothell lots, you do not need a tree retention plan or an arborist report with your ADU permit.

On a lot over 20,000 square feet, the rules kick in. A significant tree is any tree over 8 inches across, measured 4 feet above the ground, not counting alders and cottonwoods. You need a tree retention plan prepared by a qualified professional, you have to keep at least 20 percent of the total trunk inches on the lot, and any tree you keep gets a protected zone out to its dripline or 15 feet from the trunk, whichever is bigger. On a big lot, that protected zone can decide where the ADU goes.

Whatever your lot size, a big tree's roots and canopy still matter to the design even when the code does not require a report. Walk your lot and note anything large before you fall in love with a location.

Fire Flow and Sprinklers

Bothell does not have a blanket rule that every new ADU needs fire sprinklers. What it does require is a fire flow letter from your water provider with the application. Fire flow is how much water the hydrants near you can deliver to fight a fire. If the flow on your street falls short of what the fire code requires for a new building, or if fire trucks cannot get close enough to the unit, the fix is usually a sprinkler system in the ADU. On most Bothell streets this is not an issue. On a long private driveway or an older water main it can be. Ask your water provider for the letter early, because it tells you the answer before you spend money on design.

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. Bothell asks for the egress window sizes and sill heights right on the floor plans.

Basement conversions have a few other checks too:

• Ceiling height. The space needs at least 6 feet 8 inches of ceiling height, and the city wants the height written on the plan for every habitable room. 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. Bothell's current code does not ban a second front door. A walkout or outdoor stairway still has to be checked against your foundation.

Stormwater, Drainage and Grading

Stormwater is rain that runs off roofs and paving instead of soaking into the ground. Bothell follows the state stormwater manual, which adds 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, and more rules kick in at 5,000 square feet.

Under that line, Bothell still wants to see where the water goes. Every ADU site plan has to show the footing and roof drains, run separately to the connection point with a cleanout there. You have three choices: tie into an existing drain system, connect to a city storm catch basin (which needs a right of way permit), or build an on site dispersal system designed and stamped by a geotechnical engineer. A new detached unit also pays a stormwater facility charge at permit issuance.

Many ADUs add somewhere around 500 to 1,500 square feet of new hard surface, which stays under the 2,000 line. A bigger unit with a new driveway or a 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. Four providers serve Bothell lots: the City of Bothell, Alderwood Water and Wastewater District, Northshore Utility District and Woodinville Water District. Your provider decides whether a second meter is required, what the connection charges are and what the fire flow letter says. Find out who serves you before you design.
• Address assignment. The city will not accept an ADU application until the new unit has an assigned address. Apply for it with Form 54 at the start, not the end.
• Pre approved plans. Bothell does not have a pre approved ADU plan program as of October 2026. The city's draft Housing Action Plan, due for adoption in December 2026, mentions the idea but has not adopted one.

Timeline

How Long Does a Bothell ADU Permit Take?

Bothell does not publish a review target for ADU permits. The city's Development Services line is 425-806-6400 and they will give you the current estimate. What the city does publish is a list of things that have to happen before the application is even accepted, and those are where most Bothell timelines slip.

Most projects go through two or three rounds of city comments. A clean, complete first submittal is the single best way to keep that number low.

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

1. Jurisdiction and zone check. Confirm the parcel is inside Bothell city limits, which county it sits in, which zone it carries under the 2025 names, and who provides water and sewer.
2. Lot review. Your property lines, hard surfaces, slopes, streams and large trees get measured on the ground, not pulled from records. The hard surface table gets built here.
3. Design. Unit sizes and locations get tested against the setbacks, the hard surface cap, the slope and your yard. You should see options before anything is drawn for the city.
4. Reports, if required. A geotechnical report on sloped or hazard mapped lots. A tree retention plan only on lots over 20,000 square feet. Neither has a published turnaround, so get real schedules from local firms before you commit to a date.
5. Address, utility and fire flow letters. These run in parallel with design. The two week water and sewer certificate is the one to start earliest.
6. Permit submittal. The application goes in online through MyBuildingPermit.com with the site plan, floor plans, elevations, structural calculations, energy worksheets and the letters above. Plan check and planning review fees are paid here.
7. City review. The city reviews the plans and sends comments. The design team answers them and resubmits as needed.
8. Permit issued. Impact fees, permit fees and the stormwater facility charge are paid, and construction can start.

The biggest swing factors in Bothell are hard surface and critical areas. A flat lot with room under the coverage cap can move fast, because the city asks for no tree report and no parking. A lot that is already mostly paved, or one with a stream or slope on it, may wait on a redesign or a soil report before the drawings can be locked. Settling those early is the best way to keep your timeline from slipping.