Installing a prefab ADU in California happens in two places at once: the factory builds your unit while a crew prepares the foundation and utilities on your lot. On set day, a truck delivers the unit and a crane lifts it onto the foundation, usually in a single day. This guide walks through each stage, from transport permits to final inspection, with the details that decide whether set day goes smoothly.
The installation timeline
| Stage | Where | What happens |
|---|---|---|
| 1. Factory build | Factory | The unit is built and inspected in the plant to state-approved plans |
| 2. Site prep | Your lot | Grading, trenching for utilities, foundation poured or installed, inspected |
| 3. Transport | Highway | Sections travel under transport permits; wide loads may need escorts and set travel hours |
| 4. Set day | Your lot | Crane lifts each section onto the foundation; sections are joined and anchored |
| 5. Hookups and finish | Your lot | Electrical, water and sewer connected; seams, roof, siding and trim closed up; steps or decks built |
| 6. Final inspection | Your lot | City inspects foundation, anchoring, utilities and site work, then signs off for occupancy |
Transport: getting the unit to your street
Every factory-built home moves under transport rules. In our experience moving modular buildings across California, three things drive cost and scheduling:
- Width: any load over 8 ft 6 in wide needs a state transportation permit; wider sections (generally over 12 ft) also need pilot cars, which adds cost and limits the routes the hauler can take.
- Travel hours: wide-load permits can restrict when the truck may travel, so delivery often starts early in the morning.
- Last mile: narrow streets, tight corners, low wires and parked cars are the usual problems, not the highway. Walk the route from the nearest main road before set day.
Set day: the crane lift
- Crane size depends on the weight of each section and the distance from the crane to the foundation. A longer reach over a house means a bigger crane.
- Street use: the crane usually sets up in the street. Many cities require a street-use or encroachment permit and traffic control.
- Overhead lines: power lines near the lift path may need to be covered or de-energized by the utility, which must be scheduled weeks ahead.
- Neighbors: let them know the date. Cars may need to move for the day.
Foundations for a prefab ADU
A prefab ADU sits on a permanent foundation designed by a licensed engineer to the factory’s support points. Your city reviews and inspects it. Common options:
| Foundation | Best for | Tradeoff |
|---|---|---|
| Concrete perimeter (stem wall) with interior supports | Most flat lots; a traditional look with a crawlspace | More forming and concrete time |
| Engineered pier or pier-and-beam system | Sloped lots and tight access | Engineering must match soils and local seismic and wind loads |
| Slab on grade | Some panelized or site-assembled designs | Less common for volumetric modules, which are built with their own floors |
Modular, factory-built or manufactured: know what you’re buying
- Factory-built housing (modular): built to the California building code; plans approved through HCD’s factory-built housing program; carries an HCD insignia.
- Manufactured home: built to the federal HUD code and carries a HUD label. It can also be used as an ADU in California, installed on a foundation approved by your city.
Both are legal ADUs under state law. Ask your builder which one you’re getting, because it changes the paperwork and the foundation design.
What isn’t included in a factory price
Be careful comparing quotes. Factory prices often exclude site work, foundation, utility trenching and connections, crane, transport permits and escorts, steps or decks, and city fees. Get a turnkey number or a line-by-line list. See What a prefab ADU costs in California.
Frequently asked questions
How long does it take to install a prefab ADU?
The crane set itself is usually one day. Hookups, finish work and inspections typically take a few more weeks. Site prep and the foundation happen before delivery, while the unit is being built.
Do I need a crane?
Almost always for volumetric (box) modules, unless the unit can roll straight onto the foundation from the street. Panelized ADUs are assembled on site and may not need one.
Can it be installed on a hillside?
Often yes, with an engineered pier foundation and a crane sized for the reach. Access and soils decide the cost.
We’ve planned deliveries and crane sets for factory-built buildings across California for decades. Call (800) 806-7485 to talk through your site. Related: Can I build an ADU on my lot?
By Matt Banes, iModular.com. California modular building specialist with 38 years in the industry. Last updated: October 7, 2026.
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