Millcreek ADU Rules

Millcreek regulates ADUs in chapter 18.71 of its land use code. If you have seen references to chapter 19.89 or Title 19, those are from the city's earlier code; Title 18 replaced it in 2025, and chapter 18.71 has been amended since. The city's own ADU web page notes that the code is "the best place to get up to date and accurate info," and this page follows the code where the two differ.

The three kinds of ADU Millcreek recognizes

Millcreek's code defines three types, and the rules differ for each:

• Internal ADU: created within the primary dwelling or within its footprint, such as a basement apartment.
• Attached ADU: shares a wall and roof with the house, or is an added story above or below it, and has an internal doorway connecting it to the main home.
• Detached ADU: shares no common walls or roof with the house, such as a backyard cottage.

Only one ADU is allowed per property, and an ADU cannot be sold separately or subdivided from the main house. Mobile homes, manufactured homes, trailers and structures on wheels do not qualify.

Where ADUs are allowed

The property must be owner-occupied and used as a single-household residence. Internal ADUs are a permitted use. Section 18.71.040 lists attached and detached ADUs as permitted uses in the R-1 and A (agricultural) zones.

Because ADUs are a permitted use, the decision is administrative: the Planning Director determines whether an application complies with the code. The code's procedure for this does not list a public hearing.

Detached ADU standards

Detached ADUs have the most specific rules. From Table 18.71-2:

Two practical points follow from that table. First, the size limit is tied to the house: on a property with a 900 square foot home, the ADU cannot exceed 900 square feet. Second, the setback grows with height, so a taller unit needs to sit further from the property lines. Under the zone's general accessory structure rules, that works out to 12 feet 6 inches at the full 24 feet. The zone's lot coverage limit also applies; in the R-1 zones it ranges from 25 percent on the largest lots to 50 percent in R-1-3.

Other detached ADU rules in section 18.71.090:

• No building on slopes of 30 percent or greater.
• Windows on a wall within 15 feet of a neighboring home must be fixed and translucent, or be skylights.
• Entrances, parking and stairways within 15 feet of a neighboring property must be screened from view.
• No second-story balconies, and exterior stairs cannot encroach into a setback.
• Exterior lighting must point downward, with light sources not visible from neighboring properties.
• No ADU can be built within a recorded easement.

Attached and internal ADU standards

Attached ADUs have no separate size cap in chapter 18.71; lot coverage and height follow the zone. The entrance to an attached ADU must not be visible from a public street, and occupancy is limited to two adults and any number of children.

Internal ADUs are shaped mostly by state law. Utah Code 10-21-303 requires cities to allow internal ADUs in primarily residential zones and limits what they can restrict. Millcreek adds three rules: the ADU must not change the home's appearance as a single-household dwelling, it cannot have separate utility meters, and it is not allowed in a mobile or manufactured home.

Design standards

Millcreek sets specific rules for how an attached or detached ADU relates to the house:

• At least one exterior material from the main house must cover 20 percent of the ADU's facades.
• At least 50 percent of the facades must match the main house's color.
• A pitched roof is required unless the main house has a flat roof.

These rules line up with how we design anyway. Read more about designing an ADU to match the house.

Parking

Millcreek's parking table requires one stall per ADU. The city's ADU page adds that ADU parking cannot be on the street, and that a garage conversion works only if the lot can provide another space and the lost garage parking is replaced. How that applies to your driveway is one of the first things to check. See ADU parking requirements for how cities compare.

Owner occupancy and the recorded notice

The property must be owner-occupied, which the code defines as a dwelling where the owner lives as their primary residence. The application requires proof: a current Salt Lake County tax notice or recent closing statement, a government ID showing the address, and documents from at least two other listed categories. On approval, a notice of compliance is recorded with the Salt Lake County Recorder stating that the property contains an ADU and is owner-occupied. The approval lapses if the home stops being owner-occupied. Our page on ADU owner occupancy compares this with other cities.

Renting an ADU in Millcreek

Anyone renting an ADU needs a Millcreek business license. A detached ADU cannot be used as a short-term rental, which the code defines as fewer than 30 consecutive days. Millcreek also limits short-term rentals citywide: a maximum of 30 licenses per council district, a 400 foot spacing requirement and owner residency in the primary dwelling.

For internal and attached ADUs, the city's ADU page says they may operate short-term rentals, but the code reads less clearly. If short-term rental income matters to your plans, that question should be answered by Millcreek Planning in writing before you design around it. Our guide to ADU short-term rental rules covers the state and city layers.

Utilities

Separate utility meters for an ADU are prohibited. The city's utility directory lists Mt. Olympus Improvement District for sewer and several water providers depending on the address. Mt. Olympus's adopted 2026 fee schedule treats a property with an ADU as one additional residential equivalent with one added capacity fee. For a new building or added square footage, the city also asks applicants to confirm with the utility companies that service can be provided. See ADU sewer and water.

Hillside, wildfire and floodplain areas

• Sensitive Lands: a separate building pad for an ADU is prohibited where the natural slope exceeds 20 percent, and geotechnical reports can be required.
• Wildland-Urban Interface: Millcreek began administering the WUI code on January 1, 2026. Properties in the mapped area follow added construction requirements.
• Floodplain: properties in the Neff's Creek floodplain have special requirements.

Converting an existing building

An existing detached building that meets the code can be converted or expanded into an ADU if its setbacks are not reduced further and it meets building and fire codes. Buildings that do not meet current standards may still qualify through a separate process. A property with an existing guest house cannot add another detached ADU, but the guest house can be converted. See garage conversion ADUs.

For the application steps, fees and portals, see Millcreek ADU permits. For what these rules mean on a real Millcreek lot, see building an ADU in Millcreek.

Questions about Millcreek ADU rules