What Is a Prefab ADU?

A prefab ADU (also called a factory-built or modular ADU) is an accessory dwelling unit that’s constructed largely inside a factory under controlled conditions, then transported to your property and set on a permanent foundation, rather than being built entirely on-site from the ground up. It’s still a permanent, code-compliant home. It’s simply assembled somewhere other than your backyard.

The short version: A prefab ADU is built mostly in a factory and delivered to your lot on a permanent foundation, instead of being framed and finished entirely on-site. It’s a construction method, not a different type of home, and it isn’t the same thing as a manufactured or mobile home.

Prefab vs. site-built: what actually changes

The end result, a permitted accessory dwelling unit on your property, is the same either way. What changes is where the work happens.

A site-built ADU is framed, wired, plumbed, insulated, and finished entirely on your lot, one trade at a time, exposed to weather and whatever else happens on a job site over the course of construction.

A prefab ADU is built inside a factory, where temperature, humidity, and dust are controlled, materials are staged and protected, and the same crews repeat the same proven builds. Once the unit is substantially complete, it’s transported to your property and set permanently in place.

Both approaches result in a real, permitted house. The difference is process, not the destination. For a deeper side-by-side on cost, timeline, and when site-built genuinely makes more sense, see Prefab vs. Site-Built ADU: Which Is Right for You?.

How Framework First builds a prefab ADU

Framework First builds each ADU roughly 97% inside our factory in Salinas, California. Framing, electrical, plumbing, insulation, drywall, cabinetry, flooring, and appliances are all installed indoors, under one roof, before the home ever leaves the building.

While that’s happening, a separate crew is on your property pouring the foundation, so the two halves of the project run in parallel instead of one waiting on the other. By the time the home is ready to leave the factory, your foundation has already cured and is waiting for it.

On delivery day, the finished home is trucked to your property and set onto that foundation by crane, typically completed in a single day. From there, the remaining work is utility hookups and final inspection, not months of on-site framing and finish work. Depending on the model, the full process from start to move-in ready typically runs 4 to 9 months.

Prefab ADU vs. manufactured (mobile) home: not the same thing

This is the mix-up we hear most often, so it’s worth being precise about it.

A manufactured home, historically called a mobile home, is built to a federal HUD code, sits on a steel chassis, and is designed to be relocatable. It’s regulated differently than a site-built house and is often financed and taxed differently too.

A prefab ADU from Framework First is built to California building code and local jurisdiction requirements, the same code a site-built home on your lot has to meet. It’s permanently affixed to a foundation, it’s inspected both at the factory and by your local building department, and it isn’t designed to be picked up and moved later. The “factory-built” part describes where construction happens, not a different, lesser code standard.

If you’ve heard “prefab” and pictured a mobile home park, that’s the misconception worth dropping. What you’re actually getting is a permanent house built indoors.

Building a prefab ADU with Framework First

Every model Framework First builds, from 405 to 1,200 square feet, is built using this same factory process, with a project price built around a permitting budget along with plans, the foundation, the factory build, delivery, utility hookups, appliances, and final inspection. Exact permit costs are confirmed for your address in the feasibility study. You can browse the full lineup on the models page, and if you’re not sure what fits your lot yet, a feasibility study is the right first step to confirm what’s actually buildable on your property.

Frequently asked questions

Is a prefab ADU the same as a manufactured or mobile home?

No. A manufactured home is built to a federal HUD code, sits on a chassis, and is designed to be relocatable. A prefab ADU from Framework First is built to California building code, permanently affixed to a foundation, and inspected the same way a site-built home is, both in the factory and by your local building department.

How much of a prefab ADU is actually built in the factory?

For Framework First, roughly 97% of the home is built inside our Salinas factory, including framing, electrical, plumbing, insulation, drywall, cabinetry, and appliances. The remaining work happens on your property: the foundation (poured in parallel while the home is being built), delivery, crane placement, and utility hookups.

Does a prefab ADU meet the same building code as a site-built home?

Yes. A prefab ADU is inspected against the same California building code requirements as a site-built home, it’s simply inspected in two stages: once at the factory during construction, and again by your local building department after delivery and hookup.

How long does it take to build and install a prefab ADU?

For Framework First, the typical timeline is 4 to 9 months from start to move-in ready, depending on the model. Because the foundation is poured on your property at the same time the home is being built in the factory, the two biggest phases of the project happen simultaneously instead of one after the other.

Can a prefab ADU be moved later, like a mobile home?

No. Once a prefab ADU is set on its permanent foundation and connected to utilities, it’s a fixed structure on your property, not a relocatable unit. That permanence is part of what distinguishes it, and other factory-built modular homes, from manufactured (HUD-code) housing.

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Curious what a prefab ADU would look like on your own lot? Start with a feasibility study to find out what’s buildable, or browse the full model lineup to see sizes and layouts.