Utah ADU Rules 2026: SB 284 and Detached ADUs

Utah's ADU rules now have two parts. Since 2021, HB 82 has made one internal accessory dwelling unit (a unit inside the footprint of a house, such as a basement apartment) a permitted use in residential zones across the state. And since October 1, 2026, SB 284 (2026 General Session) has required larger Utah cities to allow one detached ADU on any lot of 11,000 sq ft or more that holds a single-family home, with no conditional use hearing and capped parking. Cities still set the size, height, setbacks and, if they choose, owner occupancy. Law current as of October 2026.

This guide covers what each law says, where the summaries floating around disagree, and an 800 sq ft backyard plan, with what changes if you keep a unit under the 650 sq ft parking line. For how Utah compares with the rest of the country, see our overview of ADU laws by state.

SB 284: detached ADUs since October 1, 2026

SB 284, "Local Land and Water Modifications" (Sen. Lincoln Fillmore, Rep. Jill Koford), was signed on March 18, 2026. Its detached ADU section is new Utah Code § 10-21-304 and took effect on October 1, 2026, the date by which covered cities had to have an ordinance in place. City staff reports written to implement it, such as Taylorsville's August 2026 planning report and the code drafts of Ogden Valley, Alpine and others, summarize the section the same way:

The 11,000 sq ft threshold is the number in the city ordinances implementing the law and in most summaries; one or two quoted 10,000 sq ft, apparently from an earlier draft. 11,000 sq ft is roughly a quarter acre (10,890 sq ft), so the law reaches the larger suburban lots of the Wasatch Front and Utah County rather than older urban grids.

HB 82: internal ADUs since 2021

HB 82, "Single-Family Housing Modifications" (2021), enacted Utah Code § 10-9a-530 for cities and § 17-27a-526 for counties. As enacted, it defined an internal ADU as one created within a primary dwelling, within its footprint, for long-term rentals of 30 consecutive days or longer, and said that in any area zoned primarily for residential use, an internal ADU is a permitted use. A city could not regulate one internal ADU by its size relative to the house, total lot size or street frontage.

The enrolled 2021 text (subsection 4) let a city:

• Require one additional on-site parking space, and replacement of garage or carport spaces lost to the unit.
• Require the house to keep its single-family appearance, and prohibit a separate utility meter.
• Prohibit internal ADUs on lots of 6,000 sq ft or less, in mobile homes, and where the house is served by a failing septic tank.
• Prohibit rentals of less than 30 consecutive days, and require a rental permit or license.
• Prohibit renting the unit if the house is not the owner's primary residence (the 2021 definition of "primary dwelling" is owner-occupied).
• Exclude internal ADUs from zones covering up to 25% of the city's residentially zoned area (67% in a city with a university of 10,000 or more students).

The section has been amended since 2021 and, with the rest of the municipal land use code, renumbered within Title 10 of the Utah Code in late 2025. The structure above still describes how cities apply it, but quote the current section, not the 2021 one, in any application.

Utah ADU law side by side: internal vs detached

An 800 sq ft detached ADU plan

FloorGen's 800 sq ft 2-bed ADU template is the kind of backyard unit SB 284 guarantees: a detached building 40 × 20 ft to the outside of its walls (800.1 sq ft) with 700.5 sq ft of net floor area. Two bedrooms sit at one end, a bath and laundry alcove in the middle, and an open kitchen, dining and living room at the other.

Two numbers matter in Utah. At 800 sq ft the unit is over the 650 sq ft line, so a covered city may require up to two parking spaces for it; trim it below 650 sq ft, for example to a one-bedroom about 32 × 20 ft, and the most it can ask for is one. And it fits an 11,000 sq ft lot with room to spare: a lot 85 ft wide and 130 ft deep (11,050 sq ft) has a backyard far larger than the 40 × 20 ft footprint, so the city's setbacks and coverage limit, not the plan, decide where it goes. These lot dimensions are an illustration; your plat and your city's ordinance govern.

If you already have a legally built detached garage or shop, the conversion process SB 284 requires is the other route: a 20 × 20 ft garage holds a studio of about 360 sq ft net, laid out in our garage conversion ideas.

Building code and permits

Utah adopts the International Residential Code statewide with state amendments, and a detached ADU is reviewed like a small house: habitable rooms of at least 70 sq ft and 7 ft in any direction (2021 IRC R304), ceilings of at least 7 ft (2021 IRC R305), an emergency escape opening for the sleeping area (2021 IRC R310), and smoke and CO alarms. Converting a garage also means insulating it to the energy code and dealing with the slab and the old door opening; see our garage conversion guide.

1. Confirm your city is covered and read its new detached ADU ordinance (many cities adopted theirs in the weeks before October 1).
2. Measure the lot: is it 11,000 sq ft or more by the recorded plat, and what setbacks and coverage apply?
3. Check utilities: SB 284 lets a city refuse an ADU where water, sewer or power are inadequate.
4. Draw the site plan and floor plan; how to draw floor plans for a building permit lists what to show.

FloorGen's IRC profile checks room areas and widths, ceiling heights, bedroom escape windows, stairs and door swings as you draw. It does not check Utah zoning, lot size, setbacks or parking. For how other Western states wrote their laws, compare Utah with Montana's ADU law, which sets a size formula in the statute itself, and with Arizona's casita law and Colorado's HB24-1152, which reach every single-family lot in the cities they cover.