If you're touring an Alpine home on a half-acre lot and picturing a guest cottage behind the garage for visiting parents, can you build it? As of this fall, the city's answer is yes in principle. In practice, three things decide it: how much of the lot is already covered by structures, whether the home's existing utility meter has room for a second household, and a set of rules the City Council has openly said it will revisit within months.

On September 8, 2026, the Alpine City Council approved Ordinance 2026-16 by a 3–2 vote. The ordinance adds detached accessory dwelling units to section 3.14 of the Alpine Development Code. The city acted because Utah's Senate Bill 284 required it to have a detached-ADU code in place by October 1, 2026. The council minutes record five motions that failed before one passed. That history explains why buyers comparing Alpine with other Wasatch Front suburbs should treat "ADU potential" in a listing as a question to investigate rather than a feature already included.

What the state requires and what Alpine added

The bill text the Legislature posted directs a covered city to allow a detached ADU on any lot of 11,000 square feet or larger that holds a permitted single-family home. Cities may also allow ADUs on smaller lots. The SB 284 text also lists what cities may still control: size, height, maximum lot coverage, setbacks and design. Cities can require the owner to live on the property, cap rentals at no shorter than 90 consecutive days, limit a lot to one ADU, and refuse a unit when utilities lack the capacity to serve it.

Alpine used nearly every one of those options. Detached ADUs were nothing new to the city's thinking, either. Alpine has allowed interior units, meaning basement apartments and in-home suites, under an ordinance adopted in October 2023, as the Alpine-Highland-Lehi Tribune reported in August. The city counted 178 registered accessory apartments, and each one must renew every January.

Two tables, and the lesser number wins

The code the council reviewed sets two separate limits by lot size. The first is the maximum living area of the detached unit, which must be at least 300 square feet. The second is the maximum share of the lot that all permanent structures together may cover once the ADU is added. The Planning Commission had recommended dropping the coverage limit. The council kept it for now, and the motion that passed applies whichever limit is smaller.

The last column is plain arithmetic from the percentages in the draft. It shows something a lot-size figure on a listing sheet won't. A lot just over a tier line can carry less total building than a lot just under it. At exactly 20,000 square feet, 30% coverage allows 6,000 square feet of structures. One square foot more drops the cap to 25%, or roughly 5,000 square feet. The 40,000-square-foot line works the same way, where the cap falls from 10,000 square feet to 8,800.

That matters in Alpine because the zoning is built around those exact numbers. The city's general plan defines CR-20,000 and CR-40,000 as country residential zones with 20,000- and 40,000-square-foot minimum lots, plus TR-10,000 at 10,000 square feet. New subdivisions tend to land just above the minimum. The Smooth Canyon proposal the Planning Commission approved in March 2026 had 23 lots, and the smallest was 20,017 square feet. Under the draft table, that lot sits in the 25% tier and qualifies for a 1,000-square-foot unit. A neighboring lot of exactly 20,000 square feet would get a smaller unit but a larger overall coverage allowance.

What counts toward coverage is also unsettled. The Planning Commission flagged that staff would struggle to decide what qualifies as a "permanent structure." During the debate, a participant pointed out that Alpine's existing definition of structures includes pools and horizontal surfaces. On a luxury lot with a large footprint, a three-car garage, a pool and a sport court, the coverage cap could limit an ADU well before the living-area cap does.

The meter decides more than the lot does

Once a unit fits on paper, the next limit is utilities. The draft code requires a detached ADU to share the main home's meter for water, gas and electricity, with the meter in the owner's name. If that meter lacks capacity for the added unit, the ADU is prohibited.

Sewer capacity came up in the debate as well. Minutes describe the current system as adequate but note that ADUs "could tip the scale," and that the city's 24-inch sewer lines are very expensive to install or replace. Larger homes already push on Alpine's infrastructure. In the Smooth Canyon Phase 2 review, staff noted that fire flow for bigger homes might require upsizing water lines, and that the developer had to provide irrigation shares before the final plat was recorded.

Then there's cost. The council heard that at a recent Timpanogos Special Service District board meeting, cities were encouraged to charge impact fees for ADUs. Alpine said the question still needed research and that any fee would have to be written into the ordinance. No fee amount has been published in the records reviewed for this post, and the September 22 and October 13 council agendas list no ADU items.

Owner on site, tenants for 90 days or longer

The code reviewed by the council requires that the ADU or the main home on the same lot be owner-occupied. It allows an exception for a documented temporary absence of three years or less, such as a job assignment, sabbatical, military service or a move into a care facility, as long as the owner lived there for at least a year first. A detached unit can't be rented for less than 90 consecutive days. For buyers who split time between Alpine and a second home in Southern Utah, these two rules shape how the unit can be used more than its square footage does.

Parking and form are also set. Units of 650 square feet or less need one on-site space beyond what the main house requires, and larger units need two. Setbacks follow the rules for accessory buildings, the design has to match the main home, and the roof ridgeline can't exceed 25 feet. An existing structure can be converted, but the city planner said a conversion would be prohibited if the garage is taller than 25 feet. Many Alpine homes have tall RV garages, so that height cap is a practical test for any outbuilding marketed as convertible.

The basement apartment question

The draft first barred a detached ADU on any lot that already had an internal one. The motion that passed changed this. A home with a potential internal unit can add a detached ADU, provided only one of the two is used as an accessory dwelling, with a permit and fee required. During discussion, a council member asked whether this meant an owner could have a relative living in one unit while renting the other, and the motion's sponsor said that was the intent. The two council members who voted no had pushed to table the item for better definitions covering garages, family members and properties held in trusts.

What stays open into 2027

The approved motion commits the city to working on maximum lot coverage and height limits for all residential zones over the next six months. Counting from September 8, 2026, that window runs to about March 2027. The coverage tiers above could change before a buyer who closes this winter ever submits plans. Here's what is still unresolved, based on the city's own records:

• Whether the coverage tiers stay, and what counts as a permanent structure
• Whether ADUs will carry a sewer or other impact fee, and the amount
• The amount of the registration and permit fee
• Height limits across residential zones, which also affect garage conversions

Quick answers

Does every Alpine lot qualify? The state requirement applies to lots of 11,000 square feet or more that hold a permitted single-family home. Alpine's draft table also includes a tier for lots under 10,000 square feet, and approval still depends on coverage, setbacks and utility capacity.

Can a detached ADU be used as a short-term rental? No. Under Alpine's code, rentals must run at least 90 consecutive days.

Does a separate meter fix a capacity problem? The draft requires the ADU to connect to the main home's existing meter, and prohibits the unit if that meter can't support it.

Weighing a half-acre in Alpine against an acre, or a listing that mentions room for a guest house? Olivia Bostwick can help you work out a specific lot's tier, how much of it is already covered and what its utilities can support before you write an offer. Schedule a personalized consultation and bring the address.

This article is general information, not legal, tax or financial advice. Consult a qualified professional about your situation.