Utah law already lets you build a second living unit inside your own house, and most homeowners I talk to here have no idea it is on the books. Under Utah Code 10-21-303, effective November 6, 2025, an internal accessory dwelling unit is a permitted use in any area zoned primarily for residential use, and a city may not restrict its size relative to the house, the lot size, the street frontage, or how the two spaces connect. Detached units are where the local rules bite, and St. George rewrote its version three weeks ago. Here is what the code says in each city as of September 10, 2026, with the sections so you can read it yourself.
What Utah Law Guarantees Inside the House
An internal accessory dwelling unit, meaning a separate unit created entirely within an existing single-family home, is a permitted use statewide in residential zones under Utah Code 10-21-303(1). That section was renumbered from 10-9a-530 by Chapter 15 of the 2025 First Special Session, so an older article citing 10-9a-530 is pointing at the right law under a dead number. Cities keep real authority under subsection (3): they may require one extra on-site parking space unless four are already required, ban a separate utility meter, require a rental license, prohibit rentals under 30 consecutive days, prohibit renting at all when the house is not the owner's primary residence, and refuse the unit outright if the lot is 6,000 square feet or less or the home sits on a failing septic tank.
What St. George Changed on August 20, 2026
St. George City Code 10-17A-3 governs accessory dwelling units, and the council amended it by Ordinance 2026-068, adopted August 20, 2026 and effective August 24, 2026. The change worth knowing sits in subsection (D): a lot may now carry a second accessory dwelling unit if it falls inside the downtown boundary, or if it contains at least 11,000 square feet and one unit is attached to the house while the other is detached. Two per lot is the ceiling. The city defines that downtown boundary as the area including and surrounding the Central Business District, generally bounded by Interstate 15 on the east, Bluff Street on the west, and the cliffs on the north.
The rest of the section is stricter than people expect. An owner-occupant has to live on the property, and short-term rental of an accessory dwelling unit is not permitted at all. Each unit needs its own off-street space, and tandem parking does not count. A detached unit stays smaller than the gross floor area of the house and under 25 feet tall, though a single-story one may sit zero feet off the rear and side property lines if it keeps six feet of separation from other structures. Expect an accessory dwelling permit on top of building permits, a recorded owner-occupancy deed restriction if you rent it, and a rental dwelling business license.
One wrinkle deserves a flag. Section 10-17A-7(D), unamended since 2019, still reads that only one guesthouse or accessory dwelling unit is permitted per lot of record, while the August amendment to 10-17A-3 allows two. If your plan depends on that second unit, get the city to confirm in writing which section controls before you pay for drawings. St. George planning takes those calls at 435-627-4206.
How Washington City Handles It Differently
Washington City splits the topic in two: 9-8B-7 covers detached units, last amended by Ordinance 2026-05 on January 14, 2026, and 9-8B-8 covers internal units, last amended by Ordinance 2026-21 on April 22, 2026. Either way the city caps you at one accessory dwelling unit per qualifying lot, so the second-unit door St. George just opened stays shut here.
| Detached unit rule | St. George (10-17A-3) | Washington City (9-8B-7) |
|---|---|---|
| Units per lot | 1, or 2 downtown or on 11,000+ sq ft with one attached, one detached | 1, attached or detached |
| Size cap | Less than the gross floor area of the house | 1,000 sq ft in R-1-6 to R-1-12; 1,500 sq ft in R-1-15 and larger |
| Side and rear setback | 0 ft for a one-story unit, 6 ft separation from other structures | No closer than 3 ft to any side or rear line |
| Height cap | 25 ft | 17 ft in R-1-6 to R-1-12; 20 ft at a 3 ft setback in R-1-15 and larger, to 25 ft |
| Extra parking | 1 space per unit, no tandem | 1 space to 1,000 sq ft, 2 above |
| Nightly rental | Not permitted | Not permitted except by conditional use permit |
| Verified | Code through Ord. 2026-068, Aug. 20, 2026 | Code through 2026 S-22 |
Section 9-8B-8 sets a 31-day floor on renting an internal unit, defines a kitchen down to a commercially manufactured cooking appliance, and requires a notarized deed restriction recorded before the permit application, not after. Planning and zoning answers questions at 435-656-6325.
A Casita Is Not an Accessory Dwelling Unit
This is where I see the most expensive misunderstandings. St. George's definitions section states that a casita is a guesthouse, and guesthouses live under 10-17A-7, which looks nothing like the accessory dwelling unit rules. A guesthouse runs 150 to 400 square feet, may not have a kitchen or cooking facilities, may be used only by the occupants of the main house or their nonpaying guests, and cannot be rented independently, with a deed restriction recorded before the building permit issues. It has to be new construction, so converting a shed or garage into one is off the table.
In my experience showing homes here, buyers hear the word casita in a listing and picture rental income. What usually sits behind that word is a 400 square foot guest suite with a bar sink, permitted as a guesthouse, carrying a recorded restriction against renting it. That is a fine thing to own when family visits, and it is not an income property. Before you write an offer on a home marketed with a casita or a mother-in-law suite, ask which section it was permitted under and ask to see the recorded documents. In Washington City the same question turns on whether the unit has a kitchen with a commercially manufactured cooking appliance.
One limit on all of this: code sets what the city will allow, and says nothing about what your subdivision's recorded declaration permits. That gets read one declaration at a time, ideally before you buy the lot. Current pricing sits on the St. George market summary, and the new construction page covers which builders are active if you are weighing a build.
Frequently Asked Questions
Can I build an accessory dwelling unit in St. George, Utah?
Yes, on a lot containing a single-family dwelling, if you live on the property yourself. St. George City Code 10-17A-3 permits one accessory dwelling unit on any such lot, and since Ordinance 2026-068 took effect August 24, 2026, a second is allowed on lots inside the downtown boundary or on lots of at least 11,000 square feet where one unit is attached and one detached. You need an accessory dwelling permit, building permits, a recorded owner-occupancy deed restriction if you rent it, and a rental dwelling business license.
Can you rent an accessory dwelling unit nightly in St. George or Washington City?
No. St. George City Code 10-17A-3(B) states that short-term rental of an accessory dwelling unit is not permitted, and 10-17A-7(A) applies the same ban to guesthouses. Washington City Code 9-8B-8 bars rental periods of 30 days or less for internal units, requiring a 31-day minimum, while 9-8B-7 prohibits nightly rental of accessory dwelling units unless approved by conditional use permit. Both cities require a recorded deed restriction stating the prohibition.
How big can a casita be in St. George, Utah?
Between 150 and 400 square feet of floor area, under St. George City Code 10-17A-7(F). The code defines a casita as a guesthouse, and 10-17A-7(G) prohibits a kitchen or cooking facilities, allowing only a microwave, a compact refrigerator under seven and three-quarters cubic feet and no taller than 36 inches, and a wet bar sink no wider than 12 inches. Height is capped at 15 feet for a gable roof and 12 feet for a flat roof.
Does Washington City allow two accessory dwelling units on one lot?
No. Washington City Code 9-8B-7(B)(1) allows only one accessory dwelling unit, attached or detached, on a qualifying residentially zoned lot, and 9-8B-8(C)(1) repeats that one-unit maximum for internal units. That is the clearest difference from St. George, which as of August 24, 2026 permits up to two on lots meeting its downtown or 11,000 square foot conditions.
Thinking About a Home With Room for a Second Unit?
Lot size, zone and recorded restrictions decide whether an accessory dwelling unit is realistic on a given property, and those are worth checking before you make an offer. I am happy to pull the zoning and recorded documents on any address you are considering. Call or text me at 435-200-5508, or send me a message, and browse what is listed now on the St. George homes for sale page.