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Costs and decisions

Prefab vs Custom ADU

A factory can build the walls, the roof and even the kitchen. It cannot pour your foundation, trench your sewer line, get a crane into your backyard or get your city's approval.

See What Your Property Can SupportADU Design-Build

Prefab ADUs are often presented as a way around the cost and complexity of building. Part of that is true: a unit built indoors, from a repeated design, can control some costs and some schedule risk. The part that gets left out is everything outside the factory. On a backyard ADU, that part is often where the budget and the schedule are decided.

What "prefab" and "custom" mean here

Prefab covers a range of approaches: wall panels built in a factory and assembled on site, complete modules set by crane, or a finished small building delivered in one piece. Custom means a unit designed for your lot and house and built on site, from a foundation up. Between them sit stock plans: designs drawn once and reused, built on site in the usual way.

What stays the same either way

Whichever you choose, these costs and steps apply:

  • FeasibilityZoning, lot size, setbacks, coverage, height, parking and owner occupancy rules decide whether and where an ADU can go. See can I build an ADU?
  • FoundationA permanent foundation designed for the soil and loads. Salt Lake City's ADU handbook says solid perimeter foundations are required.
  • UtilitiesSewer, water and electrical lines trenched from the house, sized for a second home. See ADU utility connections.
  • Site workClearing, grading, drainage, and restoring the yard afterward.
  • PermitsThe city's ADU approval, a building permit, and inspections. See ADU permitting.
  • FeesCity application and permit fees and any utility district charges.

What prefab adds to the list

A factory-built unit brings costs and questions of its own:

  • Delivery and setting. Modules or a finished unit have to reach the backyard, often by crane over the house, the garage or the neighbors' airspace. Power lines, trees and narrow side yards can rule it out or add cost. See ADU site access.
  • Connections and finishing. Joining modules, connecting utilities, and finishing where the factory work stops, such as porches, steps and exterior trim.
  • Code review. Ask your city's building department how it reviews and inspects the specific factory-built product, and what documentation it needs from the manufacturer, before you commit.
  • Deposits and lead times. Factories often need a commitment before the city has approved the project. Understand what you lose if approval or site conditions change.

Manufactured and mobile homes are not ADUs

A common assumption is that a manufactured home or tiny house on wheels can serve as a backyard unit. In several cities we serve, the code says otherwise:

  • Millcreek's definitions state that "Mobile homes and manufactured homes are not considered ADUs," and its detached ADU standards say trailers, mobile homes "and other portable structures, especially structures with wheels, shall not be permitted." A detached ADU must be a permanent structure.
  • South Salt Lake's code says no ADU shall be constructed within a mobile home, portable container, trailer or similar structure.
  • Cottonwood Heights' code says a mobile home or other portable structure does not qualify as an ADU.

If you are looking at a particular factory-built unit, the question to ask is whether it will be permitted and installed as a permanent building on a foundation under your city's rules.

City rules can narrow the catalog

A prefab product line comes in fixed sizes and styles. City rules may not accept all of them on your lot:

  • Size. Millcreek caps a detached ADU at the lesser of the main building's gross square footage or 1,000 square feet, and Salt Lake City caps detached ADUs at 1,000 square feet of gross floor area.
  • Height. Salt Lake City limits a detached ADU to 17 feet, with added height allowed for a pitched or flat roof if the side and rear setbacks increase to match.
  • Appearance. Millcreek requires attached and detached ADUs to use at least one of the main house's exterior materials on 20 percent of all facades, keep the house's color on at least 50 percent of facades, and have a pitched roof unless the house's roof is flat.

A custom design starts from these limits. A prefab unit has to be checked against them, and sometimes modified. See ADU design to match the house.

A middle path: pre-approved plans

Salt Lake City publishes a list of ADU standard plans that its building code staff have pre-approved, offered by private firms. They are approved plans rather than a construction method; how a given plan is built is between the owner and the firm. The city says a site review is still required to obtain a building permit, and that all applicable review and permitting fees still apply.

How to compare the two fairly

  1. Confirm what your lot can support first: size, height, placement and access.
  2. Get a complete installed price for the prefab option, including foundation, delivery, crane, utilities, site work, permits and finishing.
  3. Get a custom estimate with the same scope.
  4. Compare what each includes, what each excludes and who is responsible for coordinating the pieces.

The site and permit work is where most projects are won or lost, whoever builds the walls. Our detached ADU cost guide explains those drivers, and a feasibility review is where we check them for your lot.

Questions about prefab and custom ADUs

Is a prefab ADU cheaper than a custom one?

It can be, for the part built in the factory. But the site work, foundation, utility connections, delivery, permits and fees apply to both, and on many lots those are the largest variables. Compare complete, installed costs on your property, not the price of the unit alone.

Can I put a manufactured or mobile home in my backyard as an ADU?

Not in Millcreek, where the code says mobile homes and manufactured homes are not considered ADUs and prohibits trailers and structures with wheels as detached ADUs. South Salt Lake and Cottonwood Heights have similar exclusions. A factory-built unit that is permitted and installed as a permanent building on a foundation is a different question, which your city's building department can answer for the specific product.

Are Salt Lake City's pre-approved ADU plans the same as prefab?

No. They are building plans that the city's building code staff have pre-approved, offered by private firms. They are approved plans, not a construction method, and how a given plan is built is between the owner and the firm. The city says a site review is still required and that all applicable review and permitting fees still apply.

Is a prefab ADU faster?

Factory work can overlap with site preparation, which may shorten construction. City review, utility coordination, foundation work and final inspections still take the time they take, and factory lead times vary. We would not assume a faster overall schedule without checking each step for your project.