A prefab ADU is a backyard home built in a factory, trucked to your lot and set on a foundation, often in a single day. California law makes it easier than ever to build one: cities must decide on a complete ADU application within 60 days, most small ADUs pay no impact fees, and owner-occupancy rules for ADUs are gone. This guide covers everything you need to plan, permit, design, install and pay for a prefab ADU in California, written by people who have been setting factory-built buildings on California sites for decades.
Why California homeowners are building prefab ADUs
California cities issued 41,844 ADU permits in 2025, up from 9,005 in 2018, based on HCD Annual Progress Report data. The reasons are simple:
- Rental income or family housing without buying another property.
- State law is on your side. Approval is ministerial: if your plans meet objective standards, the city must approve them.
- Prefab saves time on site. The building is made in a factory while your foundation and utilities are prepared, so your yard is a construction zone for weeks, not months.
- Only the ADU is reassessed. Your existing home keeps its Proposition 13 base value; the county assesses only the new construction.
Prefab ADU vs. site-built ADU at a glance
| Factor | Prefab (modular or factory-built) | Site-built (stick-built) |
|---|---|---|
| Where it’s built | In a factory, then trucked and craned into place | On your lot, piece by piece |
| Time your yard is torn up | Weeks (foundation, set, hookups) | Months |
| Design freedom | Choose from a builder’s plans, with options | Fully custom |
| Who approves the building | State-approved factory plans (HCD factory-built housing program) plus a local permit for the foundation and site work | Your local building department |
| Site access | Needs a truck route and usually a crane | Needs room for crews and materials |
The five steps to a California prefab ADU
- Plan: confirm what your lot allows, including size, setbacks, utilities and access for delivery. Read Can I build an ADU on my lot?
- Design: pick a size and floor plan that fits the lot and your goal (rental, family, office). Read Prefab ADU floor plans and sizes.
- Permit: submit plans; the city has 15 business days to say if the application is complete and 60 days to approve or deny it. Read The California ADU permit process.
- Install: foundation and utilities on site while the unit is built; then delivery, crane set and finish work. Read How a prefab ADU is delivered and installed.
- Pay for it: cash, a home equity loan, a renovation loan or construction financing. Read What a prefab ADU costs in California and How to finance an ADU.
Know your rights before you start: California ADU laws in 2026 explains the state rules your city must follow.
What we bring to your ADU project
iModular has helped California buyers source, permit, deliver and set factory-built buildings since 1998. The hard parts of a prefab ADU are the same ones we handle every week on commercial modular projects: state factory approvals through HCD, transport routes for wide loads, crane access, foundations and utility hookups. We know where projects stall, and we tell you before you spend money.
Frequently asked questions
How long does a prefab ADU take in California?
Plan for several months from first design to move-in. The city must act on a complete application within 60 days. Factory production and site work can run at the same time, and the set itself is usually one day.
Do I have to live on the property to build an ADU?
No. California law permanently bars cities from requiring owner-occupancy for ADUs. A junior ADU (JADU, up to 500 sq ft inside your home) can carry an owner-occupancy rule only if it shares a bathroom with the main house.
Is a prefab ADU cheaper than a site-built ADU?
Sometimes, but not always. The factory price is predictable; site work, foundation, utilities and crane costs vary by lot. See our cost guide for real ranges.
Can I rent out my ADU?
Yes, for long-term tenants. Junior ADUs and most ADUs built under the state’s guaranteed rules must be rented for terms longer than 30 days, and cities may require the same for other ADUs.
Ready to see what fits your lot? Call (800) 806-7485. We answer seven days a week.
By Matt Banes, iModular.com. California modular building specialist with 38 years in the industry. Last updated: October 7, 2026. Legal information is general, not legal advice; confirm local rules with your city.