What Does Utah's New ADU Law Actually Require?

01What the Law Does

It is a mandate, not an encouragement. Cities do not get to opt out.

Two consequences are easy to miss. The 11,000 sq ft figure is a floor on city restrictions, not a target — subsection (1)(b) preserves a city's ability to allow detached units on smaller lots, and Salt Lake City has no minimum at all. And subsection (2)(b) requires every covered city to publish a process for converting an existing legally built accessory structure into a dwelling. That language is mandatory, and it puts garage and shop conversions on the table everywhere.

Subsection (3) then strips three levers cities used to make these projects uneconomic. No conditional use permit in a primarily residential zone. No more than two parking spaces at 650 sq ft or larger, one below. No design standards conflicting with § 10-20-618.

The CUP bar is the sleeper. A conditional use permit means a hearing, a discretionary vote, and months of carry. Removing it turns an ADU from a political process into an administrative one — worth more to a schedule than the lot-size mandate itself.

02Which Cities It Binds

"Specified municipality" is a defined term, and narrower than every city in Utah.

That captures essentially the entire Wasatch Front. We ran the two-part test against the state municipal class list for all 34 cities we cover in Salt Lake and Utah County; every one qualifies.

Outside it: unincorporated county land, which follows a different chapter entirely, and small fifth-class towns in fourth-class or smaller counties. Their existing ordinances still govern.

03Preempted vs. Preserved

This split decides whether a lot pencils. Subsection (1) governs whether you may build; subsection (4) governs how big and where. Subsection (4) is a permissive list — what a city may still do to you.

04Why Size Caps Decide the Deal

Size caps are not preempted. Subsections (4)(a)(i) and (4)(b)(i) expressly preserve city authority over size, height and lot coverage. Unlike the prohibitions, they do not expire on October 1.

This is the gap between "qualifies" and "worth pursuing." Across our measured Salt Lake and Utah County data, 73.7% of qualifying lots have room for a pad larger than the entire existing house. On those parcels the city cap, not the back yard, sets the deliverable unit.

Three things to check before drawing. Floor area or footprint is a per-city question — Taylorsville and Holladay cap footprint, most others cap floor area. The cap limits the unit, not the pad, so a two-story design delivers capped square footage on half the ground. And no cap in the code means no cap: subsection (4) is permissive, so a city is not obligated to adopt one and there is no statutory default.

05How a City Still Says No

A qualifying lot is not an approved lot. Four exits survive October 1, and three are invisible in parcel data.

06Cities Still Out of Compliance

About 20% of the qualifying lots we measure sit in a city whose published ordinance would currently refuse the unit. Every one of those restrictions conflicts with § 10-21-304 and lifts on October 1.

Cities are already moving, and landing on the statutory number rather than fighting it. Lehi cut its detached floor from 14,520 to 11,000, framed in council materials as state requirements. West Valley City enacted a detached section at 11,000 in June 2026. Herriman rewrote its ADU chapter in May. Of every city we caught mid-amendment, all landed exactly on the statutory floor.

07What Changes in Practice

On October 1 a large block of Wasatch Front lots moves from "ask permission and hope" to "meets the standards, issue the permit." The bottleneck shifts from entitlement to finding the lots.

Of the ADU permits we mined from the one Utah city that publishes them openly, 92.5% were pulled by the homeowner rather than a contractor. That is a market currently served by owner-builders and small remodelers. Homeowner-facing coverage of this law is heavy and growing, so owners are reading that they may now have a path — while very few know whether their own lot clears 11,000 sq ft, let alone whether anything fits once setbacks apply.

Lot Size Is a Filter, Not an Answer

Clearing 11,000 sq ft gets a parcel past the statutory gate and says nothing about whether a unit fits. What decides that is the rectangle left after the house, garage, shed, setbacks and front-yard exclusion are subtracted — and whether equipment and a utility trench can reach it. Those are measurements, not lookups, and the size test alone passes far more parcels than survive the rest.

Three Angles Worth Pricing Before October

• Accessory-structure conversions. Subsection (2)(b) forces every covered city to publish a conversion path. Existing slabs and roofs on lots that already clear the floor, at a lower basis and shorter schedule.
• The cities that are behind. Sandy, Orem, Highland, Saratoga Springs, American Fork and Taylorsville hold a large share of qualifying lots and a field that still believes the market is closed. The preemption date is fixed and public.
• Two-story product under generous caps. Where a city caps floor area, stacking doubles deliverable square footage on a constrained pad. Where it caps footprint — Taylorsville, Holladay — it does not.

08Sources

Every statutory quotation here is taken from the code text at le.utah.gov rather than secondary coverage. Start with the statute.

• Primary — the statute Utah Code § 10-21-304, Detached accessory dwelling units — the operative text, effective 10/1/2026.
• Primary — definitions Utah Code § 10-21-101 — the "specified municipality" test at subsection (25).
• Primary — the bill S.B. 284, Local Land Use Modifications, 2026 General Session — status, votes and every substitute version.
• Primary — enrolled text S.B. 284, 6th Substitute (PDF) — the version that carried, enacted as Chapter 166.
• Related statute Utah Code § 10-20-618 — cross-referenced by § 10-21-304(3)(d), which bars ADU design standards conflicting with it.
• Secondary — city comparison Utah ADU Builders — Utah ADU rules by city — internal vs. detached rules across jurisdictions.
• Secondary — homeowner framing Redsign — Utah just changed the rules on backyard homes — a short consumer-side summary, useful to forward to an owner.
• Secondary — contractor framing Stoneward Construction — SB 284 — notes the HOA and short-term-rental limits the statute leaves alone.

Where secondary sources conflicted with the code, we followed the code. Two live examples: several summaries describe cities as retaining broad authority over minimum lot size, which (1)(a) preempts above 11,000 sq ft; and at least one treats size caps as locally negotiable without noting that (4)(a)(i) and (4)(b)(i) affirmatively preserve them. Both change a pro forma.