What is a prefab or modular ADU?
Prefab and modular ADUs are built substantially off-site in a factory, then delivered to your property and installed on a prepared foundation. Because much of the construction happens in parallel with your site prep and permitting, on-site build time is often shorter than a fully custom detached build. It's still a detached, freestanding unit once installed — the difference is where and how it's built, not the end result.
Design options are typically chosen from a manufacturer's existing floor plans and finish packages rather than built fully custom, which is part of what makes cost and timeline more predictable up front.
Is prefab or modular right for you?
What to consider
Trade-offs worth weighing
Faster factory construction does not mean a faster permit — your city's approval timeline still applies in full, and that's frequently the longer pole in the tent regardless of build method. Site access for delivery is a hard constraint: a narrow side yard or no crane access can rule this option out entirely, so it's worth confirming early rather than after you've picked a floor plan. Design flexibility is also more limited than a fully custom detached build, since you're generally selecting from a manufacturer's existing plans rather than designing from scratch.
Frequently asked questions
No. A prefab or modular ADU is built to the same California residential building code as site-built construction — it's manufactured off-site in sections and assembled on your permanent foundation, not a manufactured home on a chassis.
Most modular ADU manufacturers offer a set of base floor plans with a range of finish and layout options, so customization is more limited than fully custom site-built construction — but many buyers find enough flexibility to fit their needs.
Because the unit is manufactured in a factory in parallel with site prep and foundation work, rather than sequentially — weather delays and on-site labor scheduling tend to have less impact on the timeline compared to fully stick-built construction.