Mill Creek allows up to two accessory dwelling units per lot, each between 400 and 1,000 square feet of gross floor area. An ADU needs one off-street parking space unless a major transit stop is within a half mile of the lot, and the application must show the water and sewer districts can serve it.
The city updated these standards in 2025 as part of the work to bring its zoning in line with the state’s ADU and middle housing laws. The rules are spread across three parts of the Mill Creek Municipal Code: the ADU section in the general provisions chapter, the residential zone chapters and the off-street parking chapter. This guide pulls them together for homeowners. To compare Mill Creek with other cities, start with our HB 1337 city comparison.
Where Mill Creek’s ADU Standards Live in the Code
The core text is MCMC 17.22.130, a short section first written in 2005 and amended several times since, most recently by Ordinance 2025-933. It covers the application, size, number of units, alley setbacks, conversions, street improvements, code compliance and utilities.
Each residential zone chapter then lists ADUs as allowed in accordance with that section, and the zone chapter supplies the setbacks, height and lot coverage. Parking comes from the table in Chapter 17.27. The code is published on the eCode360 platform; because amendments arrive throughout the year, check the online version, not a printed copy, before you design.
A Floor of 400 and a Ceiling of 1,000 Square Feet
Mill Creek is one of the few cities in the area with a minimum ADU size as well as a maximum. The gross floor area of an ADU must be at least 400 square feet and no more than 1,000 square feet.
The minimum rules out a tiny studio cottage or a small room converted with a kitchenette. The maximum sits exactly at the state floor for local caps. A one-bedroom or compact two-bedroom plan usually fits inside the range, while larger plans need trimming. Our explainer on how ADU square footage is capped in Washington describes what other cities count and exclude, which is worth raising with Mill Creek staff for garages, decks and storage.
Two Units and How They Can Be Combined
The code states that no more than two ADUs are allowed per lot. It does not limit which types, so two attached units, two detached units or one of each are available where the lot has room, in line with the statewide rule explained in our article on ADU counts per lot in Washington.
If you are also weighing middle housing, such as a duplex or cottage housing under the 2025 amendments, ask the planning department how ADUs and middle housing interact on the same lot before you commit to either path.
Fitting a Backyard Unit on an NR Lot
Many single-family homes in Mill Creek sit in residential zones such as NR, where the code sets a minimum lot size of 7,200 square feet. For a single-unit dwelling, the NR chapter sets:
Front setback: 15 feet.
Side setbacks: starting at 5 feet.
Rear setback: 20 feet or 20 percent of lot depth, whichever is less, but never less than 10 feet.
Lot coverage: 35 percent.
Under state law, ADU setbacks and lot coverage may not be stricter than those for the principal home, and Mill Creek’s own section lets an ADU sit directly on an alley property line. For a plain-language walk through these measurements, see our guide to setbacks and lot coverage for ADUs.
Small Accessory Buildings Inside a Setback
Mill Creek’s general rule for accessory buildings allows them within a side or rear setback, but no closer than 5 feet to the lot line, no larger than 400 square feet and no taller than 12 feet, with windows on walls facing a neighbor kept low. Because the ADU minimum is also 400 square feet, a unit placed in that zone has almost no room to spare. Ask staff whether they apply this rule to a detached ADU before drawing one in the rear setback.
Garages That Predate Today’s Setbacks
An ADU may be established in an existing structure, including a detached garage, even if that structure does not meet current setback or lot coverage requirements. Converting an older garage can therefore place a unit closer to a property line than new construction could go.
No Separate Height Table for Mill Creek ADUs
The ADU section has no height table of its own. In the NR zone, single-unit dwellings may reach 35 feet, and the accessory building rule limits buildings that meet principal-building setbacks to the maximum height of the zone. State law also prevents a city from setting an ADU height limit below 24 feet unless the main house faces a lower limit. Confirm with staff which figure applies to your detached design.
One Space per ADU, Zero Near Major Transit
The parking table requires one off-street space for an ADU. Within half a mile of a major transit stop, the requirement drops to zero. Under state law, stops on bus rapid transit routes count as major transit stops, and Community Transit’s Swift Orange Line has served Mill Creek since 2024, so some addresses near Swift stations may qualify. Measure walking distance to the nearest station and ask the city to confirm.
One detail is easy to miss: the parking chapter states that tandem spaces, where one car parks behind another, are not counted toward minimum requirements. A long driveway may not satisfy the ADU space on its own, so ask how the city would treat your layout.
Water and Sewer District Sign-Off Comes First
An ADU application must include documentation from the local water and sewer districts showing adequate potable water supply and sewer capacity. The code points to districts rather than a city utility, so expect an outside agency with its own forms and timelines.
Request that letter early. It can reveal whether a detached unit needs a new service, a meter or a longer sewer run, which affects both the site plan and the schedule.
Street Improvements, Occupancy and Associations
Approval cannot be tied to new sidewalks, curbs or other public street work.
Mill Creek’s ADU section says nothing about where the owner lives, and state law prohibits owner-occupancy requirements in any case. Short-term rental use is not addressed in the ADU section; ask whether other city licensing rules apply if you plan nightly stays.
Much of Mill Creek was built as a planned community, and many neighborhoods have homeowner associations with recorded covenants. Those private rules are separate from the city code and should be reviewed alongside it.
How a Mill Creek ADU Application Moves Forward
The owner of a lot or residence applies to the city’s Department of Community Development on the department’s forms, and the city keeps a file of every approved accessory unit. Expect building permit review of the full design along with zoning, utility and site review.
Mill Creek borders Everett and sits a short drive from Snohomish, and the three cities handle ADUs very differently. Our guides to Everett’s ADU rules and the City of Snohomish ADU code show the contrast. For a specific property, the United Signature team for Mill Creek can review the lot, the association documents and the design options with you.
FAQ
Would a 350-square-foot studio cottage qualify as an ADU in Mill Creek?
No. The code sets 400 square feet of gross floor area as the minimum for an ADU, so a studio that small would need to grow to at least that size before it could be permitted as one.
Can my ADU parking space be behind my garage parking in Mill Creek?
Possibly not. The parking chapter states that tandem spaces are not counted toward minimum requirements, so a space that sits behind another car may not satisfy the one-space rule. Ask staff to confirm for your layout.
Do I need a letter from my water and sewer district before applying for a Mill Creek ADU?
Yes. The application must include documentation from the local water and sewer districts showing adequate water supply and sewer capacity for the new unit.
Will Mill Creek require sidewalk or curb work as part of my ADU permit?
No. Frontage upgrades cannot be made a condition of the ADU permit. On-site utility and drainage work is a separate matter.
Is my lot near Mill Creek Town Center exempt from ADU parking?
Only when a major transit stop lies within a half mile. Proximity to shops does not count, but a nearby Swift bus rapid transit station may, so measure the walking distance and confirm with the city.