What Does a Detached ADU Cost in Utah County?

Why it costs so much more than a basement

You're building a small house. Footings and foundation, framing, roof structure and roofing, siding, windows, doors, insulation, and every interior system — then a kitchen and a bathroom on top of it.

A basement apartment skips the first seven of those. That's the entire explanation for a gap that otherwise looks like someone is overcharging.

The four site conditions that move the number most

In rough order of how much damage they can do to a budget:

• Sewer depth and distance — if the unit can't reach the sewer by gravity you need a pump or a grinder, which is both a cost and a permanent maintenance obligation
• Utility run length — every foot of trench from the house or the street for water, power, and gas is money, and it scales with how far back the unit sits
• Slope and soil — a sloped lot means more excavation, retaining, and sometimes geotechnical review; foothill lots in Alpine and Highland see this most
• Access for equipment — if a machine can't get to the back yard, work that would be done in a day gets done by hand over several

What doesn't move it as much as you'd think

Finish level. On a basement finish, the spread between builder-grade and high-end is most of the range. On a detached unit the shell, the site work, and the utilities are such a large share of the total that upgrading the kitchen and flooring shifts the number by a comparatively small percentage.

Size behaves the same way. Going from 600 to 800 square feet costs less per additional square foot than the first 600 did, because the expensive fixed items — utility connections, the kitchen, the bathroom, permits, mobilization — are already paid for. If you're building one at all, the marginal square footage is usually the best-value part of the project.

Costs people forget to budget

These aren't hidden, but they're routinely left out of the mental math:

• Impact and connection fees, which vary by city and can be substantial for a new dwelling unit
• A separate electrical service or a panel upgrade at the main house
• Site restoration — the yard the trencher crossed doesn't put itself back
• Drainage and grading so water leaves both buildings, not just the new one
• Property tax reassessment, and a landlord insurance conversation with your agent

Before you spend anything on design

Confirm the lot qualifies. Utah's SB 284 requires cities over 5,000 people to permit a detached unit on lots of 11,000 square feet or more, but that's the threshold for the guarantee — your setbacks, lot coverage limits, and utility layout determine whether there's actually somewhere legal to put a building.

We've seen qualifying lots with no viable placement once setbacks were applied. Finding that out costs an hour. Finding it out after a design deposit costs considerably more.