Costs and decisions
ADU vs Home Addition
Both add living space to your property. The difference is who the space is for, and whether they need a home of their own or a bigger share of yours.
The difference in one view
An ADU
A second, separate home
- Its own kitchen, bathroom, sleeping area and entrance
- Another household can live there independently
- City ADU rules apply: owner occupancy, parking, often a recorded notice
- Can be rented long-term, with a license, where the city allows
- Can be internal, attached or detached
A home addition
More room for one household
- Bedrooms, a bath, a bigger kitchen or living space
- Shared entrance, systems and daily life
- Zoning and building rules for the house apply, not ADU rules
- Not a separate rental unit
- Built onto or above the existing house
Start with who will live there
The clearest way to decide is to picture a normal weekday. If the new space is for your own household, more bedrooms for growing children, an office, a bigger kitchen, an addition usually makes more sense: you gain room without creating a second home you have to manage. If the space is for someone who should be able to cook, come and go and close their own door, a parent, an adult child, a caregiver or a tenant, you are describing an ADU.
Many families are somewhere in between. A parent who is independent today may need more help later. An adult child may live there for a few years and then move out. When the need will change, the flexibility of a separate unit often matters more than the cost difference.
How the approvals differ
An addition goes through zoning review for setbacks, height and lot coverage, and a building permit. An ADU goes through those and the city's ADU rules. In Millcreek, for example, a property must be owner occupied to have an ADU, and the city records a notice with the Salt Lake County Recorder when the ADU is approved. Other cities have similar rules; see ADU owner occupancy.
The legal category matters too. Under Utah law, an internal ADU is one created within a primary dwelling and within its footprint at the time it is created. Build outward with new walls and a new roof, and the unit is no longer internal. Millcreek, for example, defines an attached ADU as one that shares a wall and roof with the main house, or sits as a story above or below it, and has an internal doorway connecting to it. Its entrance may not be visible from a public right-of-way. That distinction affects fees: Utah Code 11-36a-202 bars impact fees on an internal ADU within an existing primary dwelling, and the exemption does not reach an addition-style unit outside the footprint. See detached vs internal ADU and Utah ADU laws.
How the costs differ
For the same new floor area built onto the house, much of the work is identical: foundation, framing, roof tie-in, exterior finishes matched to the existing home, and extending the house's systems. The differences are:
- KitchenAn ADU needs one. An addition may not. It is the most expensive room in any home.
- SeparationAn ADU needs fire and sound separation from the main house and its own entrance. An addition opens into the house.
- SystemsAn ADU adds a second kitchen's electrical load and plumbing, which can push the panel or water service past capacity.
- Fees and parkingAn ADU may bring an application fee, utility district charges and an added parking space. An addition usually does not.
Neither is reliably cheaper; the answer depends on what each option would include on your house. Our Utah ADU cost guide explains the drivers.
Where each one fits on the lot
An addition has to fit within the house's setbacks and the zone's lot coverage. So does an attached ADU, which in Millcreek has no separate size cap, with lot coverage set by the underlying zone. A detached ADU has its own rules, and on some lots it fits where an addition would not, or the reverse. Converting space you already have, such as a basement or garage, avoids building new floor area at all. The feasibility review compares the options on your lot.
Resale and flexibility
An addition makes the house larger, and any buyer can use the space. An ADU gives the property a second home, which some buyers value and others will not use. In several cities the ADU approval depends on the owner living on the property, so a buyer has to meet that rule, and in some cities re-apply, to keep using it. Think about who is likely to buy your house, and what they would do with each.
When an addition is the better answer
If the space is for your own household, if you do not want a second kitchen, or if the city's ADU rules do not suit your plans, an addition may be the better fit. We will tell you plainly if that is where your property points, even though ADUs are our focus.
Questions about ADUs and additions
What makes an addition an ADU?
Generally, whether it is a separate dwelling: its own kitchen, bathroom, sleeping area and entrance, so another household can live there independently. Once it is, the city's ADU rules apply, including owner occupancy and, if you rent it, a rental license. A bedroom and bath added for your own household is an addition.
Is an addition cheaper than an ADU?
Not automatically. An addition without a kitchen avoids the most expensive room, and it may share the house's systems more simply. But the foundation, structure, roof tie-in and matching the existing house cost the same either way. The comparison depends on what each option would actually include on your house.
Can I build an addition now and turn it into an ADU later?
Possibly, if it is planned for from the start: plumbing located where a kitchen could go, space for a separate entrance, fire and sound separation that can be completed later, and a layout that meets the city's ADU rules. Converting later still needs the city's ADU approval and permits.
Do you build home additions?
We focus on ADUs, including attached ADUs that are built much like an addition. If what you need is more room for your own household rather than a separate home, a contractor who focuses on additions is the better fit, and we will say so.