Basement apartment and ADU rules in Utah: what is now legal

First, the words

An ADU is an accessory dwelling unit. That is a complete second home inside your home, with its own entrance, kitchen, bathroom, and bedroom. Because it is complete, someone who is not part of your household can live in it and pay you rent. People also call it a basement apartment, a mother-in-law suite, or a granny flat.

There are two kinds. An internal ADU is built inside the walls your house already has, so a basement apartment is an internal ADU. A detached ADU is a separate building in the yard, like a small home over a garage. The new state protection is mostly about the internal kind.

What the state now protects

Utah law makes an internal ADU a permitted use in areas zoned mostly for houses. That is a big deal, because it means your city cannot block one over how big the unit is next to the house, how big your lot is, how much street frontage you have, or whether the two homes connect inside. That rule is Utah Code section 10-21-303. It took effect on November 6, 2025, and it was renumbered from the old 10-9a-530, so most of what you find online still cites the old number.

The protection is narrow on purpose. Three things all have to be true. The unit has to sit inside the footprint your house already has, with nothing new added on. It has to be rented for 30 consecutive days or longer, not by the night. And the house it sits in has to be a detached single-family home that you occupy as your own primary residence. Miss any one, and you are back to whatever your city decides on its own.

What your city can still require

Quite a lot, and this is where projects get surprised. The same law still lets a city ask for one extra on-site parking space. It can require the house to still look like a single-family home from the street. It can require a permit or a license to rent the unit, and it can ban rentals shorter than 30 days. It can say no if the lot is 6,000 square feet or smaller, or if the home is not the owner's primary residence. And it can block a separate utility meter or record a notice on your title.

Two Utah County cities get more room to say no than anywhere else. The state let cities keep internal apartments out of part of town, usually up to a quarter of the land zoned for houses. That rises to as much as two thirds for a city whose limits contain the main campus of a college with 10,000 or more students. In Utah County that means Provo, with BYU, and Orem, with UVU. In those two cities especially, check your own parcel rather than assuming from a neighbor's project.

The egress trap almost nobody sees coming

This is the most useful thing on this page. Utah has a rule that protects older bedrooms. If a basement bedroom was finished back when it met the code of the day, a small escape window in it is generally allowed to stay. You are grandfathered in.

Then the statute adds one sentence that takes it all back. Utah Code section 10-20-612 says, word for word, that the protection "does not apply to an internal accessory dwelling unit." That section was numbered 10-9a-511.5 before November 2025, so the older write-ups online still use that number.

So a bedroom that was perfectly legal while your own family slept in it stops being legal the day it becomes part of a rental unit. Nothing about the room changed. What changed is what the room is for. Every bedroom in the unit now needs a real escape window, and there is no older-house exception left to lean on. This is exactly the line a cheap bid leaves out. Cutting an opening through a concrete foundation wall and setting a window well outside is real money.

The numbers you will hear, so none of it is a surprise: a finished ceiling of about seven feet, an opening of roughly 5.7 square feet once the window is actually open, at least 20 inches wide and 24 inches tall, with the bottom no more than 44 inches off the floor. Your city's building official confirms the exact numbers for your job.

What about a backyard unit?

Section 10-21-303 does not cover detached ADUs, so those have always been the city's call. That is changing. Section 10-21-304 takes effect on October 1, 2026 and pushes larger cities to allow a detached unit on bigger lots. Utah County cities spent 2026 rewriting their detached rules to match it. If a backyard unit is what you want, treat anything you read about it as out of date until your city confirms it.

Do it in the right order

The usual way people get burned on an ADU is order of operations. They hire a designer, fall in love with a plan, and only then find out their lot is outside the overlay, or that the parking will not fit. The cheap question, "is this even allowed on my lot," should be answered first. That is the whole argument for design-build, where one team carries the zoning check, the drawings, the permit, and the build, so the answer comes before the design bill.

We lay out the full rules, city by city, on our basement apartment and ADU page. If you would rather not become a part-time code reader, we will check your address against your city's current rules for free. Not planning a rental at all? Our basement finishing page covers the same space built for your own family.