ADU rules in Salt Lake County

Detached ADUs are a permitted use, but only on single-family home lots of 7,000 sq ft or more in areas zoned primarily for residential use (R, A, Forestry, PC zones); in the PC zone the minimum drops to 6,000 sq ft if the county has approved design standards. Not permitted in townhomes, multi-family PUDs, or other attached unit types, or on lots that cannot satisfy parking, setback, or lot coverage requirements. Utah's detached-ADU mandate that took effect October 1, 2026 (Utah Code 10-21-304, an 11,000 sq ft lot floor) binds cities only; the county code for counties (Title 17, Chapter 80) has no detached-ADU section, so the county's own 7,000 sq ft rule governs here.

The code recognizes internal ADUs (IADUs) rather than attached additions: an IADU must be created within the primary dwelling and within its footprint at the time the unit is created (a garage counts if habitable and sharing a common wall). IADUs are a permitted use on single-family lots of 6,000 sq ft or more in residential zones (R, A, Forestry, PC). No category exists for an ADU added onto the house outside the existing footprint. State law makes an internal ADU a permitted use in any area zoned primarily for residential use and lets a county bar them only on lots of 6,000 sq ft or less (Utah Code 17-80-303(1), (3)(h)).

Chapter 19.15 sets no numeric size limit. Size is governed indirectly: §19.15.080 holds additions and detached ADUs to the zone's lot coverage and rear-yard coverage standards, and in residential zones accessory buildings together may cover no more than 25 percent of the rear yard (§19.76.130, repeated for the R-1 zones at §19.14.070). Detached ADU height is capped at 20 ft if the rear or side setback is under 10 ft, otherwise at the primary dwelling's height (§19.15.070), and IADUs must stay within the primary dwelling's footprint (§19.15.020). §19.15.160 bars variances from square footage requirements even though the chapter states no numeric cap.

The owner must occupy either the principal unit or the ADU, with exceptions only for medical, military service, or religious reasons for up to two years (an on-site manager must then be designated, and the owner may not collect rent on the unit the owner had occupied). Applicants must file an owner-occupancy affidavit, and a notice is recorded against the property; each new owner must re-sign and secure reauthorization. Owner occupancy cannot be varied (§19.15.160).

The owner must demonstrate, by a preponderance of the evidence, that at least one on-site space is available for the ADU beyond the existing home's required parking, which is two spaces for a single-family dwelling and may be tandem (§19.80.040). If a garage conversion creates the ADU, any spaces that had counted toward the primary dwelling's off-street parking requirement must be replaced. This matches what state law lets a county require for an internal ADU (Utah Code 17-80-303(3)(c)).

Both IADUs and detached ADUs are listed as a permitted use in qualifying zones (§19.15.030); approval runs through a building permit issued by the building official plus, before renting, a business license (§19.15.110A, §19.15.130). No conditional use hearing is required, though a building permit cannot issue until the water supplier confirms compliance with water service requirements (§19.15.110B).

ADUs may not be used as short-term rentals; any rental must be for thirty consecutive days or more (§19.15.150). A business license must be obtained before renting and maintained while rented (§19.15.130). ADUs may not be occupied by more than one family (§19.15.040) and may not be sold or subdivided separately from the single-family residence (§19.15.140D).