After years of leading ADU projects across the Wasatch Front, the same handful of mistakes show up again and again, almost always because a homeowner moved too fast on the exciting part (design) before locking down the boring part (zoning, budget, and paperwork). Here's what to watch for.
It's tempting to pick a floor plan first and figure out the rules later. In practice, your lot's size, setbacks, and city-specific size cap should shape the design, not the other way around. A 960 sq ft plan is wasted effort if your city caps detached ADUs at 800 sq ft. Get the zoning check done first, always.
An unpermitted structure isn't a shortcut, it's a liability that follows the property. It can block financing at resale, void insurance claims, and in some cases require the unit to be brought up to code or removed entirely before a sale can close. Every dollar "saved" by skipping permits tends to cost far more later.
Homeowners often budget for the visible construction and forget that trenching water, sewer, gas, and electrical service to a detached unit at the back of the lot can add real cost, especially on deeper lots or older homes with limited electrical capacity. See our utility hookup guide for what typically drives this cost up or down.
The lowest bid is sometimes lowest because corners are being cut on insurance, licensing, or crew experience. Always verify a Utah GC license directly through DOPL and ask for proof of insurance before signing anything, regardless of how competitive the price looks. Our full builder-vetting checklist covers this in detail.
Several Utah cities require an additional off-street parking space for an ADU, and depending on your lot's existing driveway and setbacks, that can be a real design constraint, not a formality. This is exactly the kind of detail that should come up during your initial site visit, not after plans are finalized.
An oddly-laid-out unit built purely around one family's current needs can be harder to rent or resell later. Standard, functional layouts (a real bedroom, a full kitchen, in-unit or accessible laundry) tend to hold rental and resale value better than highly customized one-off designs, even if the customized version feels perfect for right now.
Older homes especially can hide surprises once walls are opened or foundations are excavated. Budgeting a contingency of roughly 10% on top of your expected project cost is standard practice, and it's far better to not need it than to be caught without one.
None of these mistakes are complicated to avoid, they just require slowing down at the start. That's the entire purpose of a proper free site visit: catching zoning issues, utility realities, and design tradeoffs before you're financially committed to a plan that doesn't actually fit your lot.