ADU rules in Millcreek

Detached ADUs are a permitted use in the R-1 and A zones (MKZ 18.71.040(B)) on lots of at least 8,000 sq ft, located in the rear yard, built as a permanent structure (no trailers, mobile homes, or wheeled structures) and not on slopes of 30 percent or more. Since Ord. 26-52 (7-27-2026) the FRE, R-2, R-4 and RM use tables also list "Accessory Dwelling Unit" as permitted in legally-established single-household dwellings, although 18.71.040(B) still names only R-1 and A for attached and detached units. A property that already has a guest house does not qualify for an additional detached ADU, though a guest house can be converted. Utah Code 10-21-304 (in force since 10-1-2026) requires Millcreek to permit a detached ADU on single-family lots of 11,000 sq ft or more; the city's 8,000 sq ft minimum already goes further.

Internal ADUs are a permitted use as set forth in Utah Code 10-21-303 (MKZ 18.71.040(A) and 18.71.070); they must not change the home's single-household appearance, may not have separate utility meters, and are not allowed in a mobile or manufactured home. Attached ADUs (sharing a wall and roof with the house, or a story above or below it, with an internal doorway to the main dwelling) are permitted in the R-1 and A zones; the entrance may not be visible from a public right-of-way, occupancy is capped at two adults plus any number of children, and lot coverage follows the underlying zone. Ord. 26-65 (9-28-2026) deleted the former exterior-material, roof-pitch and color design standards for attached and detached ADUs.

A detached ADU may be no larger than the lesser of the gross square footage of the existing main building or 1,000 sq ft, and the underlying zone's lot coverage limit can reduce it further. Detached height is the lesser of 24 ft or the main building's height. There is no separate size cap for internal or attached ADUs beyond zone lot coverage, and Utah Code 10-21-303(1)(b) bars the city from limiting an internal ADU's size relative to the primary dwelling.

A property is eligible for an ADU only if it is owner occupied (MKZ 18.71.030(A)); "owner-occupied" means the owner lives in the dwelling as a primary residence (MKZ 18.97). A Compliance Determination for an existing ADU requires proof of owner occupancy and lapses if the home stops being owner-occupied, and the city records a notice at the Salt Lake County Recorder stating the property is owner-occupied. Utah Code 10-21-304(4)(c) expressly allows this requirement for detached ADUs.

Table 18.63-2 requires 1 parking stall per accessory dwelling unit, on top of the 2 spaces per unit (one covered) required for the single-household dwelling. Conversions of existing attached or detached structures must accommodate all required parking on the property (MKZ 18.71.130). The city's ADU page adds that ADU parking may not be on the street and that a garage conversion must replace the garage parking with at least one covered stall. One space is within the caps of Utah Code 10-21-304(3)(b)-(c) for detached ADUs and 10-21-303(3)(c) for internal ADUs.

Internal, attached and detached ADUs are permitted uses, not conditional uses. A new detached ADU needs a Site Plan Approval decided by the Planning Director (MKZ 18.15.030(B)) plus the city's ADU application and owner affidavit before a building permit; an existing ADU can be legalized through a Planning Director Compliance Determination (MKZ 18.15.030(E), 18.71.050). Converting a legally established noncomplying structure requires a permit from the Land Use Hearing Officer (MKZ 18.71.130). This satisfies Utah Code 10-21-304(3)(a), which bars a conditional use permit for detached ADUs in residential zones.

A detached ADU shall not be used as a short-term rental (MKZ 18.71.090(C)), meaning a stay of fewer than 30 consecutive days (MKZ 18.97); violations can lead to a lien of up to $100 per day after a 14-day cure period (18.71.120). Internal and attached ADUs may be short-term rentals only under a Millcreek STR business license, which requires the owner to live in the dwelling as a primary residence and is subject to citywide limits on the number and spacing of STRs (MKC 5.19.040, 5.19.080, 5.19.090). The city's ADU page states that long-term rentals of 30 days or more are allowed in all ADU types with an ADU compliance letter and a business license.