ADU Rules: Boise vs Salt Lake City (2026 Comparison)
Compare ADU regulations between Boise ID and Salt Lake City UT. Utah SB 174 preempts local restrictions on internal ADUs; Idaho has no statewide ADU law. Both Mountain West boomtowns.
Side-by-Side Comparison
Boise vs Salt Lake City
| BoiseIdaho | Salt Lake CityUtah | |
|---|---|---|
| ADUs Allowed? | Yes (local code only) | Yes (UT SB 174 preemption + local code) |
| State Preemption? | No — Idaho has no statewide ADU law | Yes — Utah SB 174 (2021) requires internal ADUs by-right |
| Max Detached ADU Size | 1,000 sq ft or primary dwelling floor area (whichever is less) | 1,000 sq ft (or 50% of primary dwelling) |
| Max Attached ADU Size | 50% of primary dwelling floor area | 50% of primary dwelling up to 1,000 sq ft |
| Rear Setback | 10 feet | 10 feet |
| Side Setback | 5 feet | 5 feet |
| Owner Occupancy Required? | No | No (UT SB 174 prohibits for internal ADUs) |
| Parking Required? | 1 space per ADU (waived within 0.25 miles of transit) | 1 space per ADU (reduced near TRAX light rail) |
| Estimated Permit Fees | $1,500–$6,000 total | $1,500–$6,000 total |
Key Differences
Boise and Salt Lake City are two of the fastest-growing Mountain West cities in the United States, both grappling with housing affordability pressures driven by population influx from coastal cities. Their ADU frameworks are similar in practice, but differ in state backing. Salt Lake City benefits from Utah Senate Bill 174 (effective May 2021), which requires all Utah municipalities to permit at least one internal ADU (attached or within the primary dwelling) by-right on all single-family lots, without requiring a conditional use permit. SB 174 also prohibits municipalities from requiring owner-occupancy as a condition of internal ADU approval. This state backing gives Salt Lake City property owners a legal right to internal ADU development that Boise homeowners lack — Boise's ADU allowances are purely local ordinance, subject to city council changes. Both cities have similar dimensional limits (1,000 sq ft maximum, 5 ft side/10 ft rear setbacks), comparable permit fees ($1,500–$6,000), and similar timelines (4-8 weeks). Neither city has the California-style comprehensive state ADU preemption covering all ADU types — SLC's SB 174 is narrower (internal ADUs only), though SLC's local ordinance extends allowances to detached ADUs as well. Boise's main regulatory risks: Idaho's Legislature has been less active on housing reform than Utah's, meaning Boise's ADU-permissive local code lacks state backing and could be tightened.
Cost Comparison
ADU construction costs are comparable in both cities. Boise construction typically runs $120,000–$260,000 for a new detached ADU; Salt Lake City runs $130,000–$270,000. Permit fees are similar at $1,500–$6,000 in both cities, with water/sewer connection fees as the primary variable. Neither city has California-style school impact fees. Both cities have significantly lower ADU development costs than California or Seattle.
Our Verdict
Salt Lake City has a regulatory edge due to Utah's SB 174 state preemption for internal ADUs — your right to build a compliant internal ADU is backed by state law and enforceable against any local restriction. Boise is equally permissive in practice under its local ordinance, but without state backing. For investors building ADUs in both markets, construction costs and permit fees are comparable. Both cities have strong rental demand from population growth. The tiebreaker for regulatory security goes to Salt Lake City.
Explore Each City
Frequently Asked Questions
Does Utah require all cities to allow ADUs?
Utah Senate Bill 174 (effective May 2021) requires all Utah municipalities to allow at least one internal ADU (attached to or within the primary dwelling) by-right on all single-family lots. Municipalities cannot require a conditional use permit for internal ADUs. SLC's local code also allows detached ADUs. Idaho has no equivalent state mandate — Boise's ADU allowances come from local ordinance only.
What is an internal ADU under Utah SB 174?
Under Utah SB 174, an internal ADU is a dwelling unit that is part of or attached to the primary residential structure — this includes basement apartments, attic conversions, attached units, and attached garage conversions. Detached ADUs in the backyard are not covered by the state mandate, though Salt Lake City's local code permits them as well.
Are ADU permit costs similar in Boise and Salt Lake City?
Yes, permit fees are comparable: $1,500–$6,000 total in both cities, with building permit fees based on construction valuation. Water and sewer connection fees are the primary variable in both cities. Neither has California-style school impact fees, making Mountain West cities significantly cheaper to permit than Bay Area or LA.
Can I rent an ADU in Boise or Salt Lake City without living on-site?
Yes in both cities. Boise does not require owner-occupancy for ADUs. Salt Lake City cannot require owner-occupancy for internal ADUs under Utah SB 174, and SLC's local code does not require owner-occupancy for detached ADUs either. Both cities allow property owners to rent ADUs as long-term rentals without residing on the property.
Source: PropertyZoned Editorial Research. Last verified April 5, 2026. View source