ADU Rules in Bothell

Bothell allows two accessory dwelling units on most residential lots, each up to 1,200 square feet, and a detached unit can rise to 30 feet. The city requires no off-street parking and caps ADU impact fees at half the house rate, but a lot with middle housing or critical areas gets one ADU.

Few cities in the region rewrote their ADU section as often as Bothell did between 2024 and 2025, and the result is a code that is generous on height and parking while drawing firm lines around streams, wetlands and multi-unit lots. This guide covers the current Bothell Municipal Code section on ADUs, BMC 12.14.135, along with fees, utilities and the lot types you see across the city. For a regional comparison, start with our city-by-city HB 1337 summary.

Three Ordinances in Eighteen Months

Bothell’s ADU section has been amended three times recently. Ordinance 2415 took effect in March 2024, Ordinance 2445 in December 2024, and Ordinance 2461 on July 8, 2025. The last one came with the city’s middle housing update and also removed parking minimums citywide.

Because the rules moved so quickly, guidance that is even two years old may describe smaller units, lower heights or parking requirements that no longer apply. Read the current code section, not a saved PDF.

The Two-ADU Default and Its Two Exceptions

The starting rule is simple: two ADUs are allowed per lot, attached or detached. Bothell then reduces that to one ADU in two situations.

Lots Developed With Middle Housing

If a lot is developed with middle housing, such as a duplex, triplex or cottage housing, it may have one ADU in any configuration. Owners planning to add units in stages should decide early whether the long-term plan is a house with two ADUs or a middle housing project with one.

Lots With Critical Areas or Their Buffers

A lot designated with critical areas or their buffers is limited to one ADU. In Bothell that commonly means streams, wetlands and steep slopes, and the buffer around a feature can reach well into a backyard even when the feature itself sits next door. A critical areas check is one of the first things to do on any Bothell lot near North Creek, Swamp Creek or the Sammamish River.

Floor Area: 1,200 Square Feet Plus a Conversion Allowance

New construction is capped at 1,200 square feet per ADU. Converting space that already exists can go further: the Community Development Director may sign off on a larger unit, provided it stays at or below 40 percent of the primary home’s gross floor area. That allowance suits large basements and bonus rooms in bigger homes.

Detached ADUs are also exempt from the general Bothell rule that limits accessory buildings to 5 percent of lot area. Without that exemption, a full-size cottage would not fit on many typical lots.

A 30-Foot Height Limit for Backyard Units

A detached ADU may be up to 30 feet tall, or 33 feet when it is built over an existing accessory structure such as a garage. Ordinary accessory buildings in Bothell are held to 20 feet or the height of the main building, whichever is less, so the ADU allowance is notably more generous.

Thirty feet is enough for a true two-story cottage, or a living unit stacked over a garage, with a pitched roof. On sloped lots, the measurement method and the main house’s position still decide how the second floor relates to neighbors’ windows and yards, so treat the number as a ceiling rather than a target.

No Parking Minimum Anywhere in Bothell

As of Ordinance 2461, Bothell’s parking chapter states that, except as specifically provided, there is no required minimum number of off-street parking spaces. The change applies across the city, not only near transit, and it removed what used to be a recurring obstacle for ADUs on narrow lots.

No minimum does not mean no parking. Most owners still want space for at least one car, and tenants often ask about it. The difference is that the site plan can be driven by how the household lives, not by a code count.

Half-Rate Impact Fees

Bothell has historically charged impact fees on new housing, including ADUs. The current code states that the city will not assess impact fees on an ADU that are greater than 50 percent of the fees that would be imposed on the principal unit. Ask the city for the current fee schedule when you apply, since rates are updated separately from the code.

Utilities, Septic and a City in Two Counties

Bothell sits on both sides of the King and Snohomish county line, and water and sewer service is split among several providers, including the city and independent districts. The city’s website has a tool for finding your provider, and that is the place to start, because connection rules, capacity charges and meter requirements depend on who serves your parcel.

Some older properties in annexed areas may still rely on septic systems. State law allows cities to restrict ADUs on lots without public sewer, so confirm sewer availability and any health district requirements before you design a second dwelling. A detached ADU also needs its own route for water, sewer and power, which is easier to plan when the main lines are located early.

Lot Patterns From Downtown to Canyon Park

Bothell’s housing stock spans older homes on larger lots near downtown and the hills around it, and the newer subdivisions that fill much of the north end around Canyon Park, North Creek and Thrasher’s Corner.

Older lots often have room for a detached unit behind the house and may have an existing garage that can be converted, or built over under the 33-foot allowance.

Newer subdivisions tend to have smaller rear yards, and many carry recorded covenants or homeowner association rules. A daylight basement or bonus-room conversion is often the better route there, and private covenants should be reviewed alongside the city code.

Lots that back onto greenbelts frequently contain stream or wetland buffers, which triggers the one-ADU limit and can shrink the buildable area.

A free lot feasibility report explains what a desktop review of zoning, slope, flood risk and utilities can tell you before anyone visits the site.

Filing an ADU Permit With Bothell Community Development

Bothell’s Community Development department reviews ADU projects, and applications are filed online through MyBuildingPermit, the regional permit portal Bothell shares with other cities and King County. A detached unit goes through building permit review with site, utility and, where relevant, critical areas review.

Bothell’s code also provides a path for existing ADUs that were never approved. The owner applies for city approval, and if no building permit exists, the unit must pass a building inspection showing it is structurally sound and meets code. Expect upgrades if the space was finished without permits.

Owner occupancy is no longer part of the equation: state law prohibits cities from requiring the owner to live on site, as our article on ADU owner-occupancy rules in Washington explains. To discuss a specific property, contact the United Signature team serving Bothell, which designs, permits and builds ADUs from start to finish.

FAQ

How tall can an ADU built over my garage be in Bothell?

Up to 33 feet when the unit is built over an existing accessory structure, compared with 30 feet for a new detached ADU. The garage must be able to carry the added structure, which an engineer confirms.

Why would my Bothell lot be limited to one ADU?

Either because the lot is developed with middle housing, or because it contains critical areas such as streams, wetlands or steep slopes, or their buffers. Both situations reduce the allowance from two ADUs to one.

Can a Bothell basement conversion be larger than 1,200 square feet?

Yes, with the director’s sign-off. A converted unit can pass 1,200 square feet, up to a ceiling tied to 40 percent of the house’s gross floor area, so the option mainly helps owners of larger homes.

Does a detached ADU count against Bothell’s accessory building size limit?

No. Detached ADUs are exempt from the rule that limits accessory buildings to 5 percent of lot area, although setbacks and other site standards still apply.

Which parts of Bothell are in Snohomish County?

Much of the north end, including the Canyon Park and Thrasher’s Corner areas, is in Snohomish County, while downtown and the southern neighborhoods are in King County. The same city code applies in both, but utility providers and some regional charges differ.