ADU costs
What a Basement ADU Costs
The walls, floor and roof are already paid for. What you are buying is the work that turns a basement into a separate, legal home, and that depends on the basement you have.
The short version
A basement ADU is usually one of the less expensive ways to add a unit, because the structure and most of the utilities are already there. The cost goes into four things: getting light and a legal way out, making the space tall and dry enough to live in, adding a kitchen and bathroom below grade, and separating two households under one roof.
Utah also treats internal ADUs differently from detached ones on impact fees. That is covered below.
- Egress: windows and wells cut through the foundation
- Height and moisture: ducts, beams, water and insulation
- Plumbing: drains below the slab and their slope to the sewer
- Separation: fire, sound and the entrance
- Systems: heating, ventilation and electrical capacity
Egress and light
Sleeping rooms need a way out if there is a fire, and a basement is where that is hardest to provide. If the existing windows are too small or too high, new openings have to be cut through the concrete foundation wall, with a header, a window well and drainage outside. Each one involves saw cutting, possibly structural review and exterior excavation. The number of bedrooms and where they sit on the plan drive this cost directly.
Utah has a rule that stops cities from forcing egress window changes to an existing bedroom that met code when it was finished. That rule "does not apply to an internal accessory dwelling unit" (Utah Code 10-20-606). In practice, plan on the ADU's egress being reviewed as part of the conversion, even if the basement bedrooms have been used for years. More on the code side in ADU building code.
Ceiling height and moisture
Ducts, beams and pipes that hang below the joists often decide whether a basement has enough headroom. Rerouting ductwork or plumbing is a moderate cost. Lowering a floor is a major one. Before any of that, the basement has to stay dry: signs of water, poor exterior grading or failed window wells need fixing first, because finishes over a damp wall fail. Insulation of foundation walls and the rim joist is part of the same line item.
Plumbing below the slab
A kitchen and bathroom in a basement need drains that fall to the building sewer. If the sewer line leaves the house above the basement floor, or the new fixtures are far from the existing stack, the slab has to be cut and trenched, and some layouts need a pump. Where the bathroom and kitchen go on the plan is one of the biggest cost decisions in a basement ADU, and a camera inspection of the existing line is money well spent before design.
Separation between the two homes
Two dwellings under one roof need fire separation between them, and good design adds sound control on top of the code minimum, particularly in the ceiling between the basement and the floor above. The entrance matters as well. An exterior stair down to a basement door is a significant piece of work. Using an existing interior stair with a new door and landing costs less but affects privacy for both households. In Millcreek, an internal ADU must not change the appearance of the house as a single-household dwelling, which shapes where a new entrance can go.
Heating, ventilation and electrical
The unit needs heat, fresh air, a kitchen exhaust and a bathroom fan. Whether it shares the furnace and water heater with the main house or gets its own depends on the design and on the codes that apply. On the electrical side, a second kitchen can push an existing panel past its capacity. Several cities, including Millcreek, prohibit a separate utility meter for an ADU, so both homes stay on the existing service. See ADU electrical service.
Kitchen, bath and finishes
These are the same choices as in any remodel: cabinets, counters, fixtures, flooring, lighting. They are the part of the budget you control most directly. Because the unit is small, a few decisions, such as a full kitchen versus a compact one, or a tub versus a shower, move the total noticeably.
Fees, and Utah's impact fee rule for internal ADUs
State law gives internal ADUs a specific advantage. Under Utah Code 11-36a-202, a political subdivision or private entity "may not impose an impact fee" on development activity that consists of the construction of an internal accessory dwelling unit "within an existing primary dwelling." State law defines an internal ADU as one created within a primary dwelling, within its footprint at the time it is created, and for the purpose of offering a long-term rental of 30 consecutive days or longer. It defines a primary dwelling as a detached single-family home occupied as the owner's primary residence. A basement conversion in an owner-occupied house often fits that description, but if the unit is for family rather than rent, confirm with your city and utility districts how they apply the exemption. There is no equivalent state exemption for detached ADUs.
That exemption covers impact fees only. Other costs still apply:
- City application fees. Millcreek's fee schedule lists a $250 compliance determination for an internal or attached ADU.
- Building permit fees, usually based on the project's valuation.
- Utility district charges. Mt. Olympus Improvement District's 2026 schedule, for example, treats a property with an ADU as one additional residential equivalent plus one capacity fee. Confirm with your sewer and water providers how they treat an internal unit.
- A rental license if you plan to rent it. See ADU rental income.
What state law says cities can and cannot require
Internal ADUs are a permitted use in areas zoned primarily for residential use (Utah Code 10-21-303). Cities cannot restrict their size in relation to the main house. They may prohibit a separate meter, require one extra on-site parking space, prohibit an internal ADU on a lot of 6,000 square feet or less, and require a permit or license to rent it, among other allowed limits. Each of these can add cost, parking most of all if the driveway has no room. Compare the options in detached vs internal ADU and Utah ADU laws.
How we price a basement ADU
We start by walking the basement: ceiling heights under the ducts and beams, where the sewer leaves the house, the panel, signs of moisture, the existing windows and the options for an entrance. That gives a scope and a budget that fit the actual space. It also sits alongside the Utah ADU cost guide, which covers every ADU type.
Questions about basement ADU costs
Is a basement ADU the cheapest kind of ADU?
Often it is among the least expensive, because the foundation, structure and roof already exist and utilities are close by. But not always. A basement that needs new egress openings cut through concrete, a lowered floor, or a new drain line broken into the slab can cost more than expected. The condition of your basement decides it.
Do I pay impact fees on a basement ADU?
Utah law says no political subdivision or private entity may impose an impact fee on the construction of an internal ADU within an existing primary dwelling (Utah Code 11-36a-202). A basement ADU in an existing, owner-occupied single-family home often fits the statute's definition, but confirm how your city applies it, particularly if the unit will house family rather than a tenant. Other charges can still apply, such as building permit fees, city application fees and utility district service charges, so confirm each one for your address.
Does a basement ADU need its own furnace and water heater?
Not always. Separating systems adds cost, and sharing them has trade-offs for comfort, control and noise. The building code and any Utah amendments that apply to your project decide what is required, and we work that out with the design and the building department rather than assuming.
Can the city limit how big my basement ADU is?
For an internal ADU, state law bars cities from restricting its size in relation to the main house. Cities can still apply building, health and fire codes, and some other limits allowed by statute, such as a prohibition on lots of 6,000 square feet or less, or an extra parking space.