ADU Rules in Utah County: State Law and How Each City Differs

What does Utah state law say about ADUs?

Utah has two sets of rules, one for ADUs inside the house and one for detached units in the yard.

Internal ADUs (basement and in-home apartments)

Under Utah Code § 10-21-303, an internal ADU, meaning one created within the footprint of an owner-occupied, single-family home for rentals of 30 days or more, is a permitted use in any area zoned primarily for residential use. A city cannot restrict its size relative to the house, total lot size beyond the limits below, street frontage, or internal connection. It must meet building, health, and fire codes. A city may:

• Require one extra parking space, unless the city already requires four, and replace garage spaces if the garage is converted.
• Prohibit internal ADUs on lots of 6,000 square feet or less.
• Prohibit rentals of less than 30 days.
• Require the owner to live in the home to rent the unit, and require a rental permit or license.
• Prohibit a separate utility meter, and require the house to keep its single-family look.
• Exclude internal ADUs from zones covering up to 25% of the city's residential area, or up to 67% in a city with a university main campus of 10,000 or more students.
• Record a notice on the property, and place a lien of up to $100 a day for violations that are not cured after written notice (at least 14 days for short-term rental violations, 30 days for others).

Detached ADUs (backyard cottages)

Under § 10-21-304, effective October 1, 2026 (passed as SB 284 in the 2026 session), cities of the first through fourth class, and fifth-class cities of 5,000 or more in larger counties, must allow a detached ADU on any lot of at least 11,000 square feet that has a permitted single-family home. In those cities:

• No conditional use permit can be required in a primarily residential zone.
• Parking is capped at two spaces for a unit of 650 square feet or more, and one for a smaller unit.
• There must be a process to convert a legally built accessory structure, like a garage or shop, into a detached ADU.
• The city may require owner occupancy, cap the unit at the size of the house, keep it out of the front yard and easements, prohibit rentals under 90 days, allow only one ADU per lot, and deny it where utilities lack capacity.

How do Utah County cities differ on ADUs?

Cities can be more generous than the state minimum, and many are writing new ordinances for the October 2026 law. Here is what current city documents say:

The takeaway: parking is the rule that most often decides whether an ADU works on a given lot, and it varies from one extra space to four or more in total. Confirm the current ordinance with the city before you design anything, because several cities are mid-update.

What does an ADU cost in fees, and does it raise property tax?

• Impact fees. Under Utah Code § 11-36a-202, a city or district may not charge an impact fee for building an internal ADU within an existing home. Detached ADUs can be charged impact fees, though some cities, like Spanish Fork, say they do not charge them.
• Permits and licenses. Expect a building permit, plus whatever registration or rental license the city requires, such as Spanish Fork's $60 annual registration or Provo's rental dwelling license.
• Property tax. Utah's 45% residential exemption applies to a primary residence, including a unit that is a tenant's primary residence (§ 59-2-103), so a long-term ADU rental generally keeps residential treatment. The added value of the unit will be assessed. See our Utah County property tax guide.

Buying a home with a basement apartment: what to check

1. Is it legal? Ask the city whether the unit is registered or licensed. Some cities record a notice on the property. Orem publishes a list of legal accessory apartments.
2. Does the approval transfer? In Provo, the ADU rental license expires when the property is sold. Plan to re-apply.
3. Will you live there? Most cities require the owner to live in the house or the ADU to rent it.
4. Is there enough parking under that city's rule?
5. Egress and code. Bedrooms need proper egress windows, and the unit must meet building, health, and fire codes.
6. Will the lender count the rent? Ask your lender early how ADU income will be treated for your loan.

For more on buying a rental, see how to buy your first rental property in Utah County and our guide to house hacking with an FHA loan.

Is adding an ADU worth it?

It depends on three things: what the unit costs to build, what it will rent for, and what it adds to resale value in that neighborhood. An internal ADU in an existing basement is usually the lowest-cost path, because there is no new foundation and no state impact fee. A detached ADU on a large lot can add more value and privacy, but costs more and may carry impact fees.

Run the numbers before you build: estimated cost, realistic monthly rent for a 30-day-plus rental, the owner-occupancy rule, and how long you plan to own the home.