ADU costs
ADU Cost per Square Foot: Why It Misleads
Price per square foot works reasonably well for comparing large houses. For a small second home with a full kitchen and bath, it hides more than it shows.
It is the first number most people reach for: find a cost per square foot, multiply by the size you want, and there is your budget. For an ADU, that shortcut goes wrong in three predictable ways. The average rises as the unit gets smaller. Nobody agrees on what the number should include. And the square footage itself is measured differently depending on who is counting. This page explains each problem and what to use instead.
Problem one: fixed costs do not shrink with the unit
Some parts of an ADU cost roughly the same whether the unit is compact or generous:
- KitchenCabinets, counters, appliances, plumbing and ventilation. The most expensive room per square foot in any home.
- BathroomFixtures, tile, waterproofing, a fan and the plumbing behind them.
- UtilitiesA sewer line, water line and electrical feed cost what the distance and depth dictate, not what the floor area dictates.
- MechanicalHeating, cooling and ventilation equipment has a minimum size and cost.
- Soft costsDesign, engineering, surveys and permit applications take effort whatever the size.
In a large house those costs are spread over a lot of floor area, so they barely move the average. In a small ADU they are a much bigger share of the total. The result is simple arithmetic: the smaller the ADU, the higher its cost per square foot, even when the finishes are identical. A studio can cost noticeably less in total than a two-bedroom unit and still look worse per square foot.
Problem two: nobody includes the same things
Two figures that both say "per square foot" can be measuring different projects. Before you compare them, ask what each one covers:
- Site work, excavation and grading?
- Utility trenching and connections from the house or street?
- Foundation, or only the building above it?
- Design, engineering and permit fees?
- Utility district charges and any impact fees?
- Appliances, landscaping, fencing and replacement parking?
A figure that leaves out the site and soft costs will always look better, and it will also be wrong for your project. For a new building, the site and utility work are where the estimate swings most; see detached ADU cost.
Problem three: the square footage itself varies
Even the denominator is not fixed. Builders may quote on gross area measured to the outside of the walls, or on finished interior area. City codes use their own definitions for their size caps, and they do not match each other:
- Salt Lake City counts a detached ADU's gross floor area but excludes stairs and landings to a second-level ADU, loft space with a ceiling under 7 feet, and basements used only for storage with no internal circulation to the ADU. A covered patio roof up to 120 square feet does not count toward ADU square footage either.
- Millcreek caps a detached ADU at the lesser of the main building's gross square footage or 1,000 square feet.
So the same unit can have one area on the permit, another in a builder's quote and a third in a listing. Dividing a cost by the wrong one produces a number that looks precise and means little. Our ADU size and layout page covers how the caps work.
Size thresholds where costs step up
Cost does not rise smoothly with size. Some rules change at particular sizes, and crossing one adds a cost all at once:
- Parking. Under Utah Code 10-21-304, in effect since October 1, 2026, a qualifying city may require up to two on-site parking spaces for a detached ADU of 650 square feet or larger, and no more than one for a smaller unit.
- Engineering. Sandy's permit checklist for detached accessory buildings asks for engineer-stamped structural plans if the structure exceeds 750 square feet or its walls exceed 10 feet in height.
- Geotechnical reports. In Millcreek's mapped Sensitive Lands, a geotechnical report is required for any accessory building over 1,000 square feet.
None of these makes a larger unit wrong, but each one belongs in the comparison when you are deciding how big to build.
What to use instead
There are better ways to think about ADU cost than a single average:
- Separate fixed from variable costs. Site, utilities, the kitchen and bath, and soft costs on one side; the cost of each added room on the other. That shows what extra space really costs.
- Price the property first. The site and utility work are known before design starts, and they set the base.
- Compare bids line by line. Scope, allowances and exclusions explain most differences between bids.
- Judge value by use. What matters is whether the unit does its job, for a parent, a tenant or your own future, not its average cost per foot.
The Utah ADU cost guide covers every driver by type, and ADU budget planning explains how to turn them into a working budget.
Questions about ADU cost per square foot
What is a typical ADU cost per square foot in Utah?
We do not publish one, because a single figure would mislead more than it helps. The cost per square foot of an ADU changes with its size, type and site, and different sources include different things in it. A range built from your property's actual drivers is far more useful than an average from someone else's projects.
Why does a smaller ADU cost more per square foot?
Because many of the costs do not shrink with the floor area. A small unit still needs a kitchen, a bathroom, a sewer and water connection, an electrical feed, a foundation edge, design, engineering and permits. Spread across fewer square feet, those fixed costs raise the average.
Does making the ADU bigger make it better value?
Sometimes, up to a point. Adding a bedroom to a unit that already has its kitchen and bath adds less cost than the first rooms did. But size caps, setbacks, lot coverage and parking rules limit how far you can go, and some rules step up at particular sizes. Check the limits for your lot before planning around extra space.
How should I compare two ADU bids?
Compare scope, not totals or averages. Line the bids up item by item: site work, utility connections, foundation, structure, systems, finishes, allowances, soft costs and exclusions. Differences usually show up in what one bid leaves out or assumes.