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Utah cities told to use setbacks and lot‑coverage rules to comply with new detached‑ADU law
Summary
At a Utah League of Cities and Towns webinar, Mill Creek planning director Francis Lilly and League staff walked city officials through a recent state law requiring permitted detached ADUs on qualifying lots and reviewed how cities can still regulate height, setbacks, lot coverage, parking and owner‑occupancy to manage neighborhood impacts.
At a webinar hosted by the Utah League of Cities and Towns, city planners and legal staff were advised how to interpret a recent state law that requires cities with a moderate‑income housing plan to permit detached accessory dwelling units (ADUs) on lots that allow single‑family homes and meet an 11,000‑square‑foot threshold.
Francis Lilly, assistant city manager and planning director for Mill Creek, said the statute makes detached ADUs a permitted use (not a conditional use) in qualifying zones and requires compliance with health, fire and building codes. "The good news is that within the context of state statute, there's still a whole lot of options," Lilly said, urging cities to consult their attorneys on ambiguous items.
The change narrows what cities may regulate: Lilly and Jared Tingey of the League emphasized that local governments generally cannot impose design rules such as color, materials, roof pitch or interior layout on ADUs. But they said cities retain authority over objective standards including height, size, lot coverage, setbacks from property lines and structural easements, and certain window and massing rules where those requirements relate to setbacks or building and fire codes.
Mill Creek served as the detailed example. The city requires owner occupancy of the main residence and asks applicants to certify they live there at least 183 days a year via affidavit, prohibits short‑term rentals in ADUs, limits ADUs to one per lot and requires replacement of any lost off‑street parking when a garage is converted. Lilly said Mill Creek applies the same lot‑coverage standard to all accessory structures to preserve neighborhood compatibility.
Other municipal approaches discussed during the presentation illustrate a range of options. Murray’s code sets a 10,000‑square‑foot minimum lot size for ADUs, a 20‑foot maximum height and a one‑story limit, and requires an additional street parking space. Cedar City limits ADU height to 16 feet, caps size at 800 square feet and applies the same setbacks as the main building. Logan sets accessory setbacks matching the main residence and limits ADU footprint to 50 percent of the rear yard area. Ogden’s proposed draft (as discussed in the webinar) contemplated lot minimums as small as 4,000 square feet and size limits tied to a percentage of the primary dwelling.
Speakers cautioned cities about several recurring ambiguities. Panelists debated whether rules on window opacity in a setback would be permissible under the statute; Jared Tingey advised caution and recommended asking city attorneys for opinions. They also said municipalities may require separate utility laterals if infrastructure requires it and may prohibit separate utility meters for ADUs where appropriate.
On prefabricated and modular units, the presenters said the statute appears to prevent blanket bans on modular or crane‑in ADUs so long as they meet statutory standards; cities can require foundations or permanent utility connections as a condition of compliance. Regarding trailers or mobile homes, the panel said while prefab units should be allowed if compliant, cities may use foundation or connection requirements to exclude roll‑off mobile homes.
Panelists answered attendee concerns about fire inspections and hydrant distance by urging contact with local fire districts and noting that building officials commonly conduct required inspections. They also addressed how development agreements and homeowner association rules interact with the statute: the code section noted in the presentation does not supersede land‑use restrictions in development agreements signed before 05/06/2026, but city actions must comply with the new code; HOA restrictions remain a private contract matter that cities do not enforce.
Lilly urged planners to complete any necessary ordinance updates well before the state deadlines and to document a clear local rationale for any restrictions that remain in place. "Make sure that that's done before October 1, and always consult with your city attorney and just double check what state statute is saying," he said.
The League will distribute the webinar recording, slides and a white paper with sample ordinances and links to about 20 existing city ADU codes referenced during the presentation.
