ADU Building Code in Utah: IRC Edition and ADU Amendments

Two questions decide whether an ADU gets built. The zoning question is whether the city allows it on your lot, and the building code question is whether the design keeps two households safe under one roof or on one lot. Most of what homeowners read about ADUs covers the first. This page covers the second, using only what Utah's statutes and the local building departments publish.

Which code applies

Utah adopts building codes at the state level. Utah Code 15A-2-103 adopts "the 2021 edition of the International Residential Code" (the IRC), which covers one- and two-family dwellings. The same section adopts the 2024 editions of the International Building, Existing Building, Energy Conservation, Plumbing and Mechanical Codes, and the 2023 National Electrical Code. A version of the section that takes effect January 1, 2027 keeps the 2021 IRC and changes the wildland-urban interface code to the 2024 edition.

Local building departments apply those codes. Murray's Building Division says the IRC "will remain 2021" while the other 2024 codes took effect July 1, 2026. Sandy lists the 2021 IRC with the 2024 IBC, IFC, IPC, IMC, IFGC, IEBC and IECC and the 2023 NEC. The Greater Salt Lake MSD's ADU checklist refers to the 2021 IRC with state amendments.

State ADU law points back to these codes. An internal ADU "shall comply with all applicable building, health, and fire codes" (10-21-303(2)). Under the detached ADU section that took effect October 1, 2026, a city's detached ADU rules must require compliance with building, health and fire codes (10-21-304(2)).

Utah's ADU amendments to the IRC

Utah amends the IRC in statute, and several amendments name accessory dwelling units specifically. They are worth reading closely, because they are narrower than most people assume.

The definition

Utah Code 15A-3-202(9) adds this to IRC section R202: "ACCESSORY DWELLING UNIT: A habitable living unit created within the existing footprint of a primary owner-occupied single-family dwelling." For building code purposes, then, the ADU provisions are written for units inside an existing owner-occupied home, like a basement ADU or another internal ADU. That is narrower than the zoning definition, which also includes detached units.

Separation between the units: the gypsum exception

15A-3-202(22) adds an exception to IRC section R302.3: "Accessory dwelling units separated by walls or floor assemblies protected by not less than 1/2-inch (12.7 mm) gypsum board or equivalent on each side of the wall or bottom of the floor assembly are exempt from the requirements of this section." In plain terms, the state has written a specific path for how the walls and floor between an ADU and the main home can be finished. Whether a given assembly meets it is the plan reviewer's call.

Emergency escape, alarms, air and power

R310.7 (within R310, the IRC egress section): the words "or accessory dwelling units" are added after "sleeping rooms" in an existing exception (15A-3-202(27)).
R314.2.2 and R315.2.2 (smoke and carbon monoxide alarms): "accessory dwelling units," is added after "Where alterations, repairs" (15A-3-202(31) and (32)).
M1602.2 (return air): "The discharge of return air from an accessory dwelling unit into another dwelling unit, or into an accessory dwelling unit from another dwelling unit, is not prohibited" (15A-3-204(1)).
E3601.6.2 (service disconnect): "An occupant of an accessory dwelling unit is not required to have access to the disconnect serving the dwelling unit in which they reside" (15A-3-206(1)).

We have quoted the Utah amendment text exactly. The underlying IRC sections they modify are published by the International Code Council, and the exact effect of each exception on your project is decided in plan review. We do not summarize what each one saves.

Egress: a state rule that does not help internal ADUs

Egress means a way out: a window or door from a sleeping room that someone can escape through, and that rescuers can enter by. Utah Code 10-20-606 generally keeps cities from forcing egress window changes to an existing bedroom that met code when it was finished. Section 10-20-606(3)(b) says that protection "does not apply to an internal accessory dwelling unit." (The county version is 17-79-611.)

The practical result: if a basement bedroom becomes part of an internal ADU, expect the city to look at its egress as it stands today. Local requirements reflect this. Murray's ADU building checklist lists bedroom egress, and Holladay's ADU ordinance calls for egress windows in internal ADUs for both existing and new construction. Egress windows often mean cutting a larger opening and digging a window well, which is one reason basement ADU cost varies so much from house to house.

General code concepts for any ADU

Beyond the state amendments, a few ideas come up on almost every ADU plan review:

Fire separation. Where two dwelling units share walls or floors, the assembly between them is designed to slow fire and smoke from one unit to the other. Salt Lake City's ADU handbook lists fire separation (R302) among the building code items for ADUs.
Independent systems. Salt Lake City's handbook also lists separate heating systems (M1602.2) and separate, accessible gas shutoffs, water shutoffs, sewer cleanouts and power disconnects, each "independently accessed for service from the respective dwelling unit."
Alarms. Murray's checklist calls for smoke and carbon monoxide detectors interconnected with the whole house, and GFCI protection in kitchens and baths.
Existing conditions. Converting space in an older home can expose wiring, plumbing or framing that must be brought up to current standards. How much depends on what is there.

Utility capacity is its own question. Adding a kitchen and bath can push an older electrical panel or sewer line past what it was sized for. See ADU electrical service and ADU utility connections.

Detached ADUs and above-garage units

A detached ADU is its own building, so it is reviewed as one, under the same adopted codes. Local items often add to the list. Salt Lake City's handbook sets fire access at "150 feet maximum from public right of way to any point along any wall" of the new unit. Sandy's detached accessory building checklist asks for engineer-stamped structural plans if the structure exceeds 750 square feet or walls exceed 10 feet in height. In wildfire-prone areas, Millcreek began administering the Wildland-Urban Interface Code on January 1, 2026. An ADU over a garage adds stair, structure and separation questions; see ADUs above garages.

How the code shows up in your project

Code compliance is checked twice: in plan review, before a permit issues, and in inspections during and after construction. Designing to the code from the start is cheaper than redesigning after review comments. We coordinate design and engineering with the code in view and prepare applications for the city's review. Only the building department can approve the plans. For the sequence, see the ADU permit process. For the state zoning layer, see Utah ADU laws.

Questions about the ADU building code

Which building code applies to an ADU in Utah?

Utah Code 15A-2-103 adopts the 2021 edition of the International Residential Code, with state amendments, and a version taking effect January 1, 2027 keeps the 2021 IRC. City building departments, including Murray and Sandy, list the 2021 IRC among their adopted codes. Your city's building department applies it during plan review and inspection.

Does an ADU need a fire separation from the main house?

Separation between the units is one of the main code questions for any ADU that shares a building with the house. Utah added an exception to IRC section R302.3 for ADUs separated by walls or floor assemblies protected by not less than 1/2-inch gypsum board or equivalent on each side of the wall or on the bottom of the floor assembly. Whether your design qualifies is something the plan reviewer decides.

Do basement bedrooms in an ADU need egress windows?

Plan on it. Utah law generally stops cities from requiring egress window changes to an existing bedroom that met code when it was finished, but that protection does not apply to an internal ADU. Cities such as Murray and Holladay list bedroom egress in their ADU requirements.

Can the ADU share heating and electrical service with the house?

Utah's amendments address both. One says return air discharge between an ADU and another dwelling unit is not prohibited, and another says an ADU occupant is not required to have access to the disconnect serving the unit. Local rules can still add requirements, so confirm the design with your building department.

Is a detached ADU built to a different code?

It is built under the same adopted codes, but Utah's ADU amendments define an ADU as a unit within the existing footprint of an owner-occupied single-family home. A detached ADU is a separate building and is reviewed as one, with local items like fire access, wildfire areas and structural review applying.