ADU Rules in Utah County: Alpine, American Fork, Lehi
What the 2026 rules say about accessory dwelling units (ADUs) in Utah County cities — Alpine, American Fork, Lehi — and how to build one legally.
By PrimeWest Construction · Published 2026-04-08
Utah’s 2021 ADU law (HB 82) required cities to permit at least internal ADUs — but every city implements it differently. Here’s how it shakes out in three Utah County cities. Always verify with the current city code before designing.
American Fork
Internal ADUs (within the primary dwelling) permitted by right in most zones. Detached ADUs need conditional use review. Short-term rental of ADUs is generally not allowed — they must be long-term tenancy.
Alpine
Internal ADUs permitted. Detached ADUs allowed in some zones with conditional use permit. HOA overlay often restricts further — check yours.
Lehi
Internal ADUs permitted by right. Detached ADUs allowed in lower-density zones subject to lot size and setback requirements. Parking requirements often the binding constraint.
Common requirements across cities
- Owner-occupied primary dwelling (no owner, no ADU).
- Single ADU per property maximum.
- Dedicated parking space for the ADU tenant.
- Separate entrance (for internal ADU).
- No subdividing the lot.
Typical build cost
Garage-to-ADU conversion: $85K–$160K depending on finishes and whether we’re adding a full bath + kitchenette. This is PrimeWest’s most-common ADU project.
Basement internal ADU: $120K–$200K including egress, separate entrance, and full kitchen.
Detached ADU (new build): $250K–$450K for 600–900 sqft, depending on site prep and finish level.
What usually surprises people
Utility upgrades. Most ADUs require separate electrical sub-panel, separate water shutoff, and sometimes a gas-line upgrade. Budget $8K–$18K for utility work alone on a detached ADU.
Considering an ADU? We’ll walk the property, verify what your city and HOA allow, and come back with a real fixed-bid estimate. (385) 505-4031.
Got a project in mind?
PrimeWest Construction is a licensed general contractor based in American Fork, Utah. We’ll walk your scope and come back with a real fixed-bid estimate.