Type, size, finish and site, priced the way an ADU is actually built. The first question is the one that matters most: inside the house, or on the lot. In Utah those two answers sit under different law and are usually more than a hundred thousand dollars apart.
A planning estimate, not a contractor quote. Nothing you enter is sent anywhere.
This is a planning estimate, not a contractor quote. It is built from Utah’s own accessory dwelling unit statute, from Bountiful’s published ADU requirements, and from current Wasatch Front ADU cost data. It cannot see your ceiling height, what your lot does at the back, what the panel can carry, or what is behind the basement wall — and those are the things a real price turns on. Impact fee figures are allowances: Bountiful publishes its own schedule, and it is worth confirming the current numbers with Planning on 801-298-6190 before you budget.
An ADU is priced by the square foot and then by everything the square foot does not cover. The unit itself is the predictable part. The foundation, the trench, the panel, the ceiling height and the fees are where two projects of the same size separate, and on a Bountiful bench lot the ground has an opinion about most of them.
The first answer this calculator asks for is the type, because in Utah that is not a style preference. Utah Code 10-21-303 makes an internal accessory dwelling unit a permitted use in any area zoned primarily for residential use, and exempts it from impact fees. Bountiful reviews an internal unit at staff level. A detached unit has none of that: it is a conditional use decided by the Administrative Committee, and it pays the fees like any new dwelling.
Everything after that is a decision you can make with the number in front of you, and the calculator shows each one as its own line so you can see what you are buying rather than watching a single total move.
The single biggest lever on this page. An internal unit skips the impact fees, the committee, the foundation, the roof and the trench. The same square footage detached buys a genuinely separate dwelling and costs six figures more.
One kitchen and one bathroom cost the same whether the unit is 400 sq ft or 1,200. A small unit therefore costs noticeably more per square foot, which is why per-foot figures quoted without a size attached are not much use.
Bountiful climbs the Wasatch bench. Slope decides the foundation type, the retaining, the drainage and where the entrance can sit — and it costs nothing at all on an internal unit, because the house is already standing there.
Narrow side-yard access does not change what you build, it changes how every yard of concrete and every roof truss gets there. On a tight lot it is a larger number than most people expect, and it buys nothing you can see afterwards.
A second dwelling is a second full electrical load, a second waste connection and often a second heating system. On a house built before the 1990s a panel upgrade is close to routine, and a waste line that has to be pumped rather than fall is its own line.
Impact fees are the clearest example of the fork: zero on an internal unit by statute, and a real allowance on anything detached or attached. Design, survey and plan review scale with how much drawing the path actually needs.
Send a description of the property, roughly what you are picturing, and who the space is for. Every plan starts with your lot — the ceiling height, the fall of the ground and what the services can carry decide more than the floor plan does.
Broadly: an internal or basement unit runs about $60,000 to $155,000, a garage conversion about $60,000 to $170,000, an attached addition about $130,000 to $280,000, and a detached unit about $180,000 to $400,000 and up. Per square foot that is roughly $100 to $250 internal, $125 to $275 for a garage conversion, $225 to $375 attached and $250 to $450 detached, all in.
You are building a whole small house rather than finishing part of an existing one: a foundation, a roof, an exterior envelope, and a trench carrying power, water and sewer out to it. On top of that a detached unit is not exempt from impact fees and, in Bountiful, is a conditional use rather than a staff review. Those last two are law rather than construction, and they are why the gap is bigger than the building work alone suggests.
Utah's internal accessory dwelling unit statute exempts them. That is one of the strongest arguments for looking at the basement before the back yard, and it applies specifically to a unit created within the primary dwelling.
One ADU per property. You must occupy one of the two units, and you cannot rent both — that would make it a duplex. An internal unit needs its own entrance or a common area with doors between the units, bedrooms need egress, and smoke and CO detectors are required. Internal units are approved by city staff; detached units go to the Administrative Committee as a conditional use.
Roughly three to five months on site for an internal unit or a garage conversion, six to nine for an attached addition and seven to twelve for a detached unit. Add permit review on top: about four to eight weeks for an internal unit and six to twelve for a detached one, since the detached path includes a committee decision.
Yes — design, survey and plan review appear as their own lines, and they scale with the type. There is a separate ADU design calculator on this site if you want to look at that fee on its own, including what a feasibility answer costs before you commit to anything.
That depends on your street, your lot and what the unit is for, and nobody can answer it responsibly from a web page. What can be said is that an unpermitted unit is worth considerably less than a permitted one, because it is harder to insure, harder to finance and harder to sell. If a unit is going in, getting it on the record is part of the value.
No. It is a planning estimate meant to get you to a realistic budget and to show which decisions actually move it. A real number comes from someone standing on your lot, looking at the ceiling height, the panel and the fall to the sewer.