Six tools, one for each part of the work. Every one of them asks the same question first, because in Utah it is worth more than any other answer: is the unit going inside the house, or on the lot? The two sit under different law, different review and different fees.
A planning estimate, not a contractor quote. Nothing you enter is sent anywhere.
Most ADU advice treats a basement conversion and a back-garden unit as two versions of the same idea. In Utah they are not. They sit under different law, they take different routes through the city, and they pay different fees — and on the same square footage the gap is usually well over a hundred thousand dollars.
Utah Code 10-21-303 makes an internal accessory dwelling unit — one created within the primary dwelling — a permitted use in any area zoned primarily for residential use. A city may set conditions: it can refuse one on a lot smaller than 6,000 sq ft, require that the design not alter the appearance of the house, require extra parking, require a licence to rent, and prohibit rentals shorter than 30 days. What it cannot do is charge impact fees:
Internal accessory dwelling units are not subject to impact fees.
A detached unit has none of that protection. It exists entirely at the discretion of local ordinance, and in Bountiful that means a conditional use requiring Administrative Committee approval rather than the staff review an internal unit gets — plus the impact fees any new dwelling pays.
| Internal ADU | Detached ADU | |
|---|---|---|
| Legal status in Utah | Permitted use under Utah Code 10-21-303 | No state protection — local ordinance only |
| Review in Bountiful | Reviewed and approved by City Staff | Conditional use, Administrative Committee approval |
| Impact fees | Exempt by statute | Charged as for any new dwelling |
| Foundation, roof, trench | Already there | All new |
| Typical all-in cost | $100 – $250 per sq ft | $250 – $450 per sq ft |
| Typical time on site | 3 – 5 months | 7 – 12 months |
| Lot size question | State lets a city refuse below 6,000 sq ft | Bountiful looks for around 8,000 sq ft buildable |
None of this means a detached unit is the wrong answer. It buys a genuinely separate dwelling with its own front door and no shared wall, which is worth a great deal to some households and nothing at all to others. It only means the question should be asked deliberately, and asked first — because it moves the number more than every finish decision put together. Confirm current requirements with Bountiful City Planning, 801-298-6190, 795 South Main Street.
Some conditions apply whichever route you take, and they are the ones most likely to change whether an ADU makes sense for a household rather than what it costs.
A property gets one accessory dwelling unit. Not two, and not one plus a converted garage.
The owner must occupy one of the two units. Bountiful records a deed restriction to that effect.
Renting out both units would make the property a duplex rather than a house with an ADU, which is a different thing entirely in zoning terms.
An internal unit needs its own entrance, or a common area with doors between the two units.
Bedrooms need egress from doorways and windows, and smoke and carbon monoxide detectors are required.
An additional off-street space for the ADU, on top of what the house already needs. This is the one garage conversions run into.
The conditions above are published by Bountiful City on its own Accessory Dwelling Units page. The size, lot-area and accessory-footprint figures quoted elsewhere on this site come from published Davis County ADU guidance rather than from the city’s own page, so treat those as a starting point and confirm them directly. Ordinances change, and a calculator is not a substitute for a phone call to Planning on 801-298-6190.
Three site conditions move an ADU estimate here more than any material choice, and all three are things you can check yourself before anyone visits.
A lot that drops away changes the foundation type, the retaining and the drainage, and decides where a front door can sit. It is the first thing that separates two otherwise identical quotes for a detached unit — and it costs nothing at all on an internal one.
Access does not change what you build, it changes how it gets there. Where a machine will not fit down the side, the trench is hand-dug and the material is barrowed, and the labour on identical work rises sharply.
The single go/no-go on an internal unit. Measure from the slab to the underside of the joists, not to the existing ceiling — a floor build-up and a new ceiling both eat into it, and it is the one dimension that can end the cheapest route.
A clean, typical specification at 750 sq ft — standard finish, one bedroom, a flat lot and services in reach — so the types can be compared against each other rather than against a pile of options. Change any answer in a calculator and the number moves, which is the entire point of them.
| Route | Typical range | What decides where it lands |
|---|---|---|
| Internal, inside the house | $104,000 – $137,000 | Ceiling height, egress, what the panel can carry, and whether the waste can fall |
| Garage conversion | $138,000 – $182,000 | The slab, the ceiling height, and replacing the parking it removes |
| Attached addition | $186,000 – $245,000 | The roof tie-in, the foundation match, and the age of the house |
| Detached unit | $223,000 – $294,000 | The slope, the side-yard access, the trench length, and the impact fees |
| Design alone, permit-ready | $10,000 – $27,000 | Type, whether the house is already drawn, and whether a survey exists |
| Feasibility answer only | about $2,300 – $5,300 | The cheapest money on the project, and the only spend that can save all the others |
On an identical 750 sq ft at an identical finish, going detached rather than internal costs roughly $135,000 more. Some of that is a building you would not otherwise have built. A meaningful part of it is simply that one route is a permitted use exempt from impact fees and the other is a conditional use that is not.
Tell us where the property is, roughly what you are picturing, and who the space is for. The useful first conversation is not about finishes — it is about the ceiling height, the fall of the ground and what the services can carry.
Yes, and there is nothing to sign up for. Nothing you enter is sent anywhere and nothing is stored. The numbers are worked out in your browser as you change the answers.
They are planning estimates, not quotes. They are built from Utah's own ADU statute, from Bountiful's published ADU requirements, and from current Wasatch Front ADU cost data. They cannot see your ceiling height, your slope, what your panel can carry or what is behind your basement wall, and those are the things a real price turns on.
Because in Utah they are not two versions of the same project. 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. A detached unit gets neither protection: in Bountiful it is a conditional use decided by the Administrative Committee, and it pays the fees. On the same square footage the gap is usually well over a hundred thousand dollars.
The city publishes that there may be one ADU per property, that you must occupy one of the two units, and that you cannot rent both — renting both 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 reviewed and approved by city staff; detached units are a conditional use requiring Administrative Committee approval.
There are two different thresholds and they get confused. Utah's statute lets a city refuse an internal unit on a lot smaller than 6,000 sq ft. Bountiful's published bar for a detached unit is a buildable lot of around 8,000 sq ft, with the combined footprint of accessory structures limited to roughly 10 per cent of the lot as a permitted use or 15 per cent as a conditional use. Confirm the current figures with Bountiful Planning on 801-298-6190 before designing around either.
Utah's internal ADU statute says internal units are not subject to impact fees. That exemption is one of the strongest reasons to look at the basement before the back yard. It applies to a unit created within the primary dwelling — a detached building in the back garden is not that, whatever it is used for.
Utah's internal ADU statute is written around long-term rental of 30 consecutive days or longer, and it allows a city to prohibit rentals shorter than that. Bountiful permits short-term rental within an approved ADU with a separate permit. It is worth settling before you build, because the answer changes what the unit is worth to you.
Because a single figure would be false precision. Two ADUs of the same size on the same street can differ by six figures depending on whether the unit goes inside or outside, what the ground does, and what the services can carry. The range is the honest answer at this stage. A firm number comes from somebody standing on the lot.