Detached vs Internal ADU

A basement apartment and a backyard cottage can house the same person, but the law treats them very differently. Internal ADUs sit inside a state framework that limits what cities can require. Detached ADUs have mostly been left to each city, and a new state law changes part of that on October 1, 2026.

How Utah law defines each type

State law defines an accessory dwelling unit as "a habitable living unit added to, created within, or detached from a single-family dwelling and contained on one lot or parcel." It then splits out two categories:

• Internal ADU: created "(a) within a primary dwelling; (b) within the footprint of the primary dwelling ... at the time the internal accessory dwelling unit is created; and (c) for the purpose of offering a long-term rental of 30 consecutive days or longer" (Utah Code 10-21-101(16)). A basement ADU in an existing home is the typical example. See internal ADUs for other layouts.
• Detached ADU: "an accessory dwelling unit that is not attached to or within a primary detached single-family dwelling and located on the same lot or parcel" (10-21-101(9)). A backyard cottage or a converted detached garage fits here. See detached ADUs.

The "primary dwelling" in the internal definition is a detached single-family home "occupied as the primary residence of the owner of record." It includes a garage if the garage is habitable space connected to the house by a common wall. So the state's internal ADU rules are built around an owner-occupied single-family home.

The attached ADU in between

An ADU built as a new addition, sharing a wall with the house but outside its original footprint, does not fit neatly into either state definition. Cities handle it differently. Millcreek's code names three types (internal, attached and detached) and describes an attached ADU as sharing a wall and roof with the house, with an internal doorway. Holladay's ADU checklist counts a unit that shares a wall with the primary home as internal. Salt Lake City's code uses only internal and detached. South Salt Lake's and Cottonwood Heights' codes do not say how a new addition is classified, so that has to be confirmed with the city. Attached ADUs covers the design side.

Internal ADUs: state protections in 10-21-303

Utah Code 10-21-303 is the core internal ADU statute for cities (17-80-303 is the county version). In any area zoned primarily for residential use, "the use of an internal accessory dwelling unit is a permitted use." A city also "may not establish any restrictions or requirements for the construction or use of one internal accessory dwelling unit within a primary dwelling," including rules on the unit's size relative to the house, total lot size, street frontage or internal connectivity.

The statute then lists what a city may do. It may:

• prohibit a separate utility meter;
• require that the unit not change the home's appearance as a single-family dwelling;
• require one additional on-site parking space, unless its code already requires four off-street spaces;
• require a permit or license to rent the unit;
• prohibit internal ADUs on lots of 6,000 square feet or less, or where the home is served by a failing septic tank;
• prohibit rentals of less than 30 consecutive days;
• prohibit renting the unit if the home is not the owner's primary residence; and
• record a notice with the county recorder that the dwelling contains an internal ADU.

Every internal ADU "shall comply with all applicable building, health, and fire codes." That is where most of the real design work sits, especially separation between units and bedroom egress. See ADU building code. The full statute is summarized on Utah ADU laws.

Detached ADUs: set city by city

For detached units, the rules that apply today come from each city's code. Here is how the cities we work in compare on the basic question of where a detached ADU is allowed:

Size, height and setback limits vary just as widely. See ADU lot size requirements and ADU setback requirements.

What S.B. 284 changes on October 1, 2026

S.B. 284 (2026 General Session) creates Utah Code 10-21-304, which took effect October 1, 2026. It requires a "specified municipality" to permit a detached ADU "on any lot or parcel that is 11,000 square feet or larger and contains a single-family dwelling, if the single-family dwelling is a permitted use." Specified municipalities are defined by city class, with a population test for smaller cities in larger counties. Towns are never included. The city's regulation may not:

• require a conditional use permit for a detached ADU in a primarily residential zone;
• require more than two on-site parking spaces for a unit of 650 square feet or larger, or more than one for a smaller unit.

The same section lets cities regulate structure size, height, maximum lot coverage, setbacks and design consistent with the house. Cities may also prohibit a detached ADU that is larger than the house, sits in a front yard or recorded easement, or is rented for less than 90 consecutive days. They may require the owner to live on the lot and allow only one ADU per lot. The statute is new, and whether a given city is covered is a question for that city. Cities may amend their own codes in response, so check the review date on each page and confirm with your city.

Impact fees: a real difference

Utah Code 11-36a-202 says a political subdivision or private entity "may not impose an impact fee" on the construction of an internal ADU "within an existing primary dwelling." There is no matching exemption for detached ADUs in state law. Two points follow:

• The exemption refers to an existing primary dwelling. If the internal ADU is part of a new home, ask the city how fees apply.
• For a detached ADU, whether city, water or sewer impact fees apply is a local question, and most of the cities here have not published an ADU-specific answer. Ask before you budget. See ADU sewer and water and detached ADU cost.

Practical differences that follow from the rules

Cities also treat the two types differently in day-to-day rules:

• Size. Salt Lake City sets no maximum floor area for an internal ADU but caps a detached one at 1,000 square feet. South Salt Lake has no size cap for internal ADUs and limits external ones to the lesser of 50 percent of the house (not counting the garage) or 1,000 square feet.
• Parking. Cottonwood Heights requires one extra stall for an internal ADU (at least three total) and two extra stalls for a detached one. See ADU parking requirements.
• HOAs. State law bars HOA rules from prohibiting internal ADUs, but no statute protects detached ADUs from HOA restrictions. See ADU HOA rules.

Construction differs too. An internal ADU raises questions of ceiling height, egress and separation inside an existing house. A detached ADU needs a foundation, a new structure and utility runs across the yard. ADU cost in Utah compares those drivers.

Which one fits your property

It depends on the lot (size, shape, rear yard depth and access), the house (a usable basement or not) and your goals (privacy, family, rental). A lot below a city's detached minimum may still hold an internal ADU. We check both options against your city's current code in the ADU feasibility review.

Questions about detached vs internal ADUs