On October 1, 2026, a state law takes effect that changes what most Utah Valley cities are allowed to say no to.
The short version circulating locally is that Utah "legalized backyard cottages." That is not quite what happened. The Legislature created a floor, applied it to a defined class of cities, attached a lot-size trigger, and then wrote a long list of things cities may still require and a short list of things they may not. The gap between the headline and the statute is where most of the money and most of the disappointment will be found.
This reads the section as enacted.
The short version
Utah Code § 10-21-304, enacted by S.B. 284 in the 2026 general session.
Effective October 1, 2026.
11,000 square feet. The lot must be at least that, and must already contain a single-family dwelling that is a permitted use.
Detached only. This is a separate regime from internal accessory dwelling units.
No conditional use permit may be required in a primarily residential zone.
Parking capped at two spaces, or one under 650 square feet.
Owner occupancy may still be required. So may design consistency, setbacks, size limits and a 90-day minimum rental term.
What an accessory dwelling unit is, and why "detached" is the whole story
An accessory dwelling unit is a second, self-contained home on a lot that already has a primary house. It has its own kitchen, its own bathroom, and its own entrance. Utah law has treated two versions of this very differently.
The older regime concerns internal accessory dwelling units — a unit created within the primary dwelling and within its existing footprint, for long-term rental of 30 consecutive days or longer. That definition now sits in the definitions section of Chapter 21 of Title 10. Provo's ordinance implements it, and our guide to Provo zoning walks through the city's conditions in detail: owner occupancy, one per dwelling, a maintained interior connection, a single visible front door, and a parking standard.
The new regime concerns detached units. A backyard cottage. A converted garage. A standalone structure with no interior connection to the house at all. Until now, whether you could build one was almost entirely a question of local ordinance, and the answers across Utah Valley ranged from permitted to functionally prohibited.
Section 10-21-304 changes the default for detached units in covered cities. It does not touch the internal-ADU rules, and it does not merge the two. If you are converting a basement, you are still in the older regime and this article is not about your project.
Which cities this actually binds
The statute applies to a specified municipality. That term is defined in the same chapter, and the definition does real work:
a city of the first, second, third, or fourth class; or a city of the fifth class with a population of 5,000 or more, if the city is located within a county of the first, second, or third class.
Utah Code § 10-2-301 sets the classes purely by population. First class is 100,000 or more. Second class is 65,000 up to 100,000. Third class is 30,000 up to 65,000. Fourth class is 10,000 up to 30,000. Fifth class is 1,000 up to 10,000. Below 1,000 is a town.
The practical consequence for Utah Valley is that the first prong sweeps in essentially every city of any size along the corridor, because the fourth-class floor is 10,000 residents. Provo, Orem, Lehi, American Fork, Spanish Fork, Springville, Pleasant Grove, Saratoga Springs, Eagle Mountain, Payson, Highland, Vineyard and their peers are all comfortably above that line. The second prong — fifth-class cities of 5,000 or more in a larger county — is what matters for the smaller municipalities.
Two cautions. First, classification is not automatic on a census estimate: under § 10-2-302 a municipality retains its classification until it is changed by a certificate from the lieutenant governor. Second, towns under 1,000 residents are outside both prongs entirely. If you are in a small municipality, confirm the class rather than assuming it.
The trigger: 11,000 square feet and a single-family home
Subsection (1)(a) is the operative sentence:
A specified municipality shall adopt a land use regulation that permits a detached accessory dwelling unit 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 on the lot or parcel.
Three conditions, all of which must hold.
The lot is 11,000 square feet or larger. That is roughly a quarter of an acre and then some — 43,560 square feet is an acre, so 11,000 is about 0.25 acres. This is the provision that decides whether the rest of the section applies to you, and it is the one to check first.
The lot contains a single-family dwelling. A vacant lot does not qualify. The unit is accessory to something.
That single-family dwelling is a permitted use. Not a conditional use, not a legal nonconforming use, but permitted in the zone.
Subsection (1)(b) then makes clear that the floor is a floor and not a ceiling: nothing prohibits a municipality from permitting detached units on lots smaller than 11,000 square feet. A city that wants to be more generous may be.
This threshold interacts with Utah Valley's development history in an uneven way. Post-war and newer subdivisions on the benches and in the south county frequently clear 11,000 square feet. The dense older grids closer to the universities frequently do not. If you own on one of the historic Provo grids, measure before you plan — and be aware that lot area is a recorded figure, not something to eyeball from a fence line. Our guide to fences, sheds and setbacks explains why the fence is very often not the boundary.
What the city must do
Subsection (2) imposes two affirmative requirements on the ordinance a city adopts.
The regulation shall require that a detached accessory dwelling unit comply with all applicable building, health and fire codes. Nothing in this statute exempts anyone from the building code, and nobody should read a zoning mandate as permission to skip a permit. Provo's own thresholds for what triggers a permit are covered in our building permits guide.
The regulation shall include a process for the owner of a legally constructed accessory structure to convert that structure into a detached accessory dwelling unit, subject to applicable dwelling and accessory structure setback requirements and to the building, health and fire codes. This is the garage-conversion pathway, and the requirement that a process exist is meaningful: a city cannot satisfy the statute by permitting new construction while leaving conversions in administrative limbo.
Note the word "legally" in that provision. A shed or garage built without a permit is not a legally constructed accessory structure, and the conversion pathway does not launder it. Utah Valley has a substantial stock of unpermitted outbuildings, and this section does not help their owners.
What the city may not do
Subsection (3) is the list of prohibitions, and it is short.
No conditional use permit may be required if the proposed unit is located in a primarily residential zone. This is the most valuable line in the section. A conditional use process means a public hearing, a discretionary body, conditions attached case by case, and months of calendar. Moving the use to permitted status converts an uncertain political question into an administrative one.
No more than two on-site parking spaces may be required for a unit of 650 square feet or larger.
No more than one on-site parking space may be required for a unit smaller than 650 square feet.
No design standards that conflict with Section 10-20-618.
The parking caps are worth dwelling on because parking requirements have historically been the quiet way to make an ADU ordinance unusable. A four-space requirement on a lot with a single driveway is a prohibition wearing a permit's clothing. Capping the requirement at two — and at one for small units — removes that lever. It also creates a genuine design incentive to build under 650 square feet.
What the city may still require, which is a great deal
Subsection (4) is much longer than Subsection (3), and reading them side by side is the fastest way to calibrate expectations.
A municipality may require the unit to conform to land use regulations governing structure size, dimension, height and maximum lot coverage. It may require conformance to setbacks, and the statute expressly permits those setbacks to account for proximity to property lines and other structures, easements, window orientation, massing "or other elements." It may require the unit to be designed consistently with the design of the single-family dwelling.
A municipality may prohibit the unit from being larger than the main house, from sitting within a public utility easement or other recorded easement, from being located in a front-yard area, and from being rented for less than 90 consecutive days.
A municipality may require owner occupancy — that the owner reside in either the main house or the accessory unit.
A municipality may require replacement parking on-site when a detached garage is converted and the main house's spaces were inside it.
A municipality may prohibit more than one accessory dwelling unit per lot.
And a municipality may prohibit the unit outright on utility grounds: where it will not have adequate access to a required project-improvement utility service — sanitary sewer, culinary water, electrical, storm water — or where a system-improvement utility service it must connect to lacks sufficient capacity to support the addition.
That last one deserves emphasis. Capacity is not a formality in a valley that has grown as fast as this one. Whether a given sewer line or culinary main can take another connection is an engineering question answered by the city, and it is a lawful basis for refusal. Anyone planning around this statute should ask the utility question early, not after design fees are spent.
What the section does not disturb
Subsection (5) carves out three things.
It does not supersede a land use regulation that governs a detached accessory building that is not a dwelling unit. Your shed rules survive intact.
It does not supersede prohibitions or restrictions on detached accessory dwelling units contained in a development agreement signed by a municipality on or before May 6, 2026. This is a hard date, and it matters in the master-planned communities that have proliferated in northern Utah County and along the west side of the lake. If your subdivision was built under a development agreement that restricts accessory units, and that agreement predates the cutoff, the state mandate does not reach it.
And it does not supersede any other land use regulation or administrative action relating to detached accessory dwelling units that is not otherwise prohibited by law.
There is a fourth constraint the statute does not mention because it does not need to: private covenants. A homeowners association's declaration is a contract among owners, not a land use regulation, and a state law directed at municipalities does not rewrite it. If your CC&Rs prohibit second dwellings, § 10-21-304 does not help you. Our HOA guide covers how those documents are enforced in Utah Valley and what an owner's realistic options are.
The timeline, and what to expect this autumn
The effective date is October 1, 2026. Cities that lack a qualifying ordinance are obliged to have one by then.
In practice, adoption is uneven. Some Utah Valley cities already permit detached units on larger lots and will need only minor amendments. Others will be drafting under deadline, and the shape of the ordinance — particularly whether owner occupancy is imposed, whether the 90-day floor is adopted, and how design consistency is defined — will vary city to city. Those choices are exactly the ones the statute leaves open, so expect real divergence across the corridor rather than a single valley-wide rule.
The practical sequence for a homeowner is unglamorous:
- Confirm the lot area from the county record, not from the plat you remember.
- Confirm the zone and that single-family is a permitted use there.
- Read your city's adopted ordinance once it exists — not the statute, the ordinance.
- Ask the utility capacity question in writing. Get the answer in writing too.
- Check for a development agreement on your subdivision, and check your CC&Rs.
- Then talk to a designer.
Steps one through five cost nothing but time. Step six is where money starts leaving.
Why the Legislature did this
The stated frame is housing supply. Utah Valley has added population faster than it has added units for most of two decades, and accessory units are the least disruptive way to add a home to an existing neighborhood: no land assembly, no new street, no new subdivision, existing infrastructure.
The counter-frame, which local governments have made consistently, is that accessory units land their costs on systems that were sized for something else — parking, sewer capacity, school enrollment, street width. Subsection (4)(f) is the Legislature conceding the strongest version of that argument, which is why the utility carve-out is the one exception that lets a city say no outright.
Whether the mandate produces many units is an open question. The 11,000 square foot threshold excludes a meaningful share of the older housing stock. Owner occupancy, if adopted, excludes the investor case. The 90-day floor, if adopted, excludes the short-term rental case. What remains is the homeowner who wants to house a parent, an adult child, or a long-term tenant on a large lot — which may well be the population the Legislature had in mind.
The short version, again
The mandate is real, it is dated, and it is narrower than the summary. It converts a discretionary permission into a permitted use on large lots in most Utah Valley cities, caps the parking lever that has historically killed these projects, and guarantees a conversion pathway for legally built garages. It leaves owner occupancy, size, setbacks, design and the short-term rental question with your city, and it leaves development agreements signed on or before May 6, 2026 and private covenants entirely alone.
Check the lot size first. Everything else follows from that number.
Financing, briefly
A detached accessory dwelling unit is a construction project, and the financing routes are the ordinary ones: cash, a home equity line, a cash-out refinance, or a construction loan. What is worth knowing in advance is that appraisers treat a detached unit inconsistently, and the appraised value added is frequently less than the cost to build. Anyone underwriting the project on an assumption that it pays for itself in equity should test that assumption with a local appraiser before committing.
Related Guides
- Provo Zoning Explained — the internal-ADU regime and the city's own conditions
- Building Permits in Provo — what triggers a permit and what does not
- Fences, Sheds and Setbacks in Utah Valley — why every city answers differently
- HOA Guide for Utah Valley — private covenants and how they are enforced
- New Construction vs. Resale in Utah Valley — the trade-offs, side by side
Frequently Asked Questions
What does S.B. 284 actually require for detached ADUs in Utah?
Which Utah Valley cities have to comply?
Can my city still require owner occupancy for a detached ADU?
Does the city have to let me build if my lot is under 11,000 square feet?
How many parking spaces can a city require for a detached ADU?
Can a city make me get a conditional use permit for a detached ADU?
Can a city block a detached ADU because of sewer or water capacity?
Can I rent a detached ADU on Airbnb once the law takes effect?
