Building an ADU in Utah: what state law allows

Internal ADUs: allowed by state law

Utah Code 10-21-303 says that in any area zoned primarily for residential use, "the use of an internal accessory dwelling unit is a permitted use." The section was renumbered from 10-9a-530 in 2025.

An internal ADU is one created within a primary dwelling and "within the footprint of the primary dwelling" when it is created, for long-term rental of 30 consecutive days or longer. A primary dwelling is a detached single-family home that "is occupied as the primary residence of the owner of record." A garage counts as part of the home if it is habitable space and shares a common wall with the house.

Apart from the limits listed in the next section, a city "may not establish any restrictions or requirements" for one internal ADU, including rules about its size relative to the house, total lot size, street frontage or internal connectivity. Rules on architectural elements must be consistent with those for single-family homes, including in historic districts. The ADU must still meet building, health and fire codes. [1][4][5]

What cities can still require for an internal ADU

State law lets a city:

• prohibit a separate utility meter;
• require that the ADU not change the home's appearance as a single-family dwelling;
• require one additional on-site parking space (not if the city already requires four), and require replacement of garage or carport parking that the ADU takes over;
• prohibit an internal ADU in a mobile home;
• require a permit or license to rent the ADU;
• prohibit internal ADUs in zones covering up to 25% of its primarily residential area (67% in a city with the main campus of a university of 10,000 or more students), except newer units on plats approved on or after October 1, 2021;
• prohibit an internal ADU if the home is served by a failing septic tank or the lot is 6,000 square feet or less;
• prohibit rentals of less than 30 consecutive days;
• prohibit renting the ADU if the home is not the owner's primary residence.

So whether you can rent out a basement apartment depends on your city's ordinance, even though the state makes the use itself permitted. [1]

Detached ADUs: the 2026 law (SB 284)

SB 284, "Local Land and Water Modifications," was signed by the governor on March 18, 2026. Most of the bill took effect May 6, 2026, but the detached ADU section, Utah Code 10-21-304, took effect on October 1, 2026.

Under it, a "specified municipality" must adopt a land use regulation that permits a detached ADU "on any lot or parcel that is 11,000 square feet or larger and contains a single-family dwelling," if the house is a permitted use there. A specified municipality is a city of the first, second, third or fourth class, or a fifth-class city of 5,000 or more people in a first-, second- or third-class county. Cities may also allow detached ADUs on smaller lots.

The city's rules must require building, health and fire code compliance and must include a process to convert a legally built accessory structure into a detached ADU. [2][3][4][6]

What cities can and can't require for a detached ADU

In covered cities, the detached ADU rules may not:

• require a conditional use permit in a primarily residential zone;
• require more than two on-site parking spaces for an ADU of 650 square feet or more, or more than one for a smaller ADU;
• include design standards that conflict with Utah Code 10-20-618.

The rules may:

• apply the city's limits on structure size, height and lot coverage, and its setbacks;
• require the ADU's design to be consistent with the house;
• prohibit an ADU larger than the house, in an easement, or in the front yard;
• prohibit renting it for less than 90 consecutive days;
• require the owner to live in the house or the ADU;
• require replacement of parking lost when a detached garage is converted;
• prohibit more than one ADU on a lot;
• prohibit the ADU if utilities lack access or capacity.

The section does not override ADU bans in development agreements a city signed on or before May 6, 2026. [2]

Garage conversions

A garage attached to the house by a common wall counts as part of the primary dwelling if it is habitable space, so it can hold an internal ADU. The city may require you to replace any parking spaces lost when the ADU is created in a garage or carport.

For detached garages, covered cities must offer a way to convert a legally built accessory structure into a detached ADU, subject to setbacks and building, health and fire codes. They may require you to replace required parking that was in the garage. See our garage conversion guide. [1][2][4]

Enforcement: liens and recorded notices

If an owner breaks the internal ADU rules, a city may put a lien on the property of up to $100 for each day of violation, but only after written notice and a chance to fix the problem: at least 14 days for illegal short-term rental, and at least 30 days for other violations. You can file a written objection within 14 days and get a hearing. If you fix the violation in time, the city may not impose a lien, penalty or fee.

A city that issues an internal ADU building permit or rental license may record a notice with the county recorder saying the home contains an internal ADU that may only be used under the city's land use rules. [1]

A bill that did not pass: HB 398 (2025)

HB 398, "Auxiliary Housing Amendments," would have amended the owner-occupancy provisions in the internal ADU law. It was not considered in committee, its enacting clause was struck, and it was filed as a bill not passed on March 7, 2025. The owner-occupied "primary dwelling" definition remains in current law. [4][5][7]

If you disagree with your city

Utah's Office of the Property Rights Ombudsman handles land use disputes through training, mediation and arbitration, and advisory opinions. You can reach it at (801) 530-6391 or toll free at (877) 882-4662. [8]

Where local rules differ

Cities write their own ADU codes within these limits, and some allow more than the state requires. Salt Lake City, for example, allows one internal, attached or detached ADU, with no minimum lot size. See our Salt Lake City ADU guide.

For the general process, see our guides to the ADU permit process, ADU costs and what an ADU is. [9]

Utah laws to know

• Utah Code 10-21-303 (internal ADUs; formerly 10-9a-530)

  Makes one internal ADU a permitted use in primarily residential areas and lists the only things a city may require or prohibit. [1][5]

• SB 284 (2026), Utah Code 10-21-304 (detached ADUs)

  From October 1, 2026, requires larger cities to permit a detached ADU on single-family lots of 11,000 square feet or more and limits parking and permit rules. [2][3][6]

• Utah Code 10-21-101 (definitions)

  Defines accessory, internal and detached ADUs, primary dwelling and the specified municipalities that must follow the detached ADU rule. [4]

Questions people ask