ADU rules in Mapleton

An accessory apartment may be permitted in a detached building only on a lot of at least 11,000 square feet (about a quarter acre). Ordinance 2026-09, adopted June 3, 2026, cut that threshold from 21,780 square feet (half an acre) to match Utah's new detached-ADU statute. The front wall of the detached structure must sit at least 10 feet behind the front wall of the primary dwelling, and the structure must also meet the city's accessory-building standards in section 18.84.230, which cap accessory buildings at 20 percent of the rear yard, require at least 3-foot side and rear setbacks below 15 feet of height, and set maximum heights that rise with lot size. Unlike the internal-unit subsection, the detached subsection does not list eligible zones.

Internal accessory apartments, as defined in Utah Code Annotated 10-21-101, are allowed in all residential zones except the SDP-1 zone (the Harvest Park development) and the R-2-B zone, on a legal lot of record of at least 6,000 square feet. The unit may not be designed in a way that changes the primary building's appearance as a single-family dwelling, and its entrance or exit must be on the side or rear. The code regulates only internal and detached types; section 18.08.010 defines an accessory apartment as a subordinate portion of a single-family dwelling. Ordinance 2026-09 refreshed the statutory cross-reference here from the repealed Utah Code 10-9a-511.5 to 10-21-101 without changing the local requirements.

A detached accessory apartment's maximum interior floor area is 1,000 square feet on lots of 11,000 to 87,119 square feet, 1,200 square feet on lots of 87,120 to 130,679 square feet, and 1,400 square feet on lots of three acres or more. Ordinance 2026-09 deleted the separate cap of 40 percent of the primary dwelling's square footage that formerly applied whichever was smaller; the City Council struck it at adoption on June 3, 2026 after a resident showed the rule limited an owner with a 1,500 square foot house to a 600 square foot unit. The internal-unit subsection sets no floor-area cap, and internal units are by definition built within the primary dwelling's existing footprint.

Either the primary dwelling or the accessory apartment must be occupied by a full-time resident property owner as shown on the Utah County tax assessment rolls. The section is titled Owner Occupied Accessory Apartments, and its provisions do not apply at all if only family members, as defined in section 18.08.010, reside at the residence. Utah law expressly leaves this choice to cities: 10-21-303(3)(j) allows a municipality to bar renting an internal unit where the home is not the owner's primary residence, and 10-21-304(4)(c) allows an owner-residence requirement for detached units.

The designated stall must be paved with asphalt or concrete or another hard surface that will not generate dust or track mud onto city streets. It may not sit in a garage unless at least two other garage stalls remain available for the primary unit, and it may not sit directly behind a required stall for the primary dwelling. No more than one designated stall may be located in the front yard setback or in a side yard setback adjacent to a street.

Before construction or occupancy the owner files the official accessory apartment permit application with the Community Development Department and pays the administrative fee set in that department's fee schedule. No public hearing or conditional use process is specified, and accessory apartments appear in no zone's permitted or conditional use list; they are regulated entirely through this supplementary section. Each kitchen also needs a building permit under section 18.84.415, which exists to enforce section 18.84.410. Since Ordinance 2026-09, a land use decision is appealed to an Appeals Hearing Officer rather than to the Planning Commission or City Council, by written notice filed with the City Recorder within 10 business days.

Section 18.84.425 prohibits short-term rentals in all zones, defining one as a residential dwelling unit or part of it used for temporary overnight stays of fewer than 30 consecutive days. Section 18.84.410(C) limits each lot to one accessory apartment. No minimum lease term or other long-term rental restriction appears in the ADU section, though the owner-occupancy requirement effectively limits who may rent out a unit.