Prefab ADU vs site-built: a decision matrix for California homeowners

"Prefab" covers several different things, and the difference matters more than the marketing label. Under California's Factory-Built Housing Law, factory-built housing is a dwelling or building component manufactured offsite so that its concealed parts cannot be inspected at the site without disassembly; it excludes mobilehomes, recreational vehicles and commercial modulars (Health & Safety Code § 19971). A manufactured home built to the federal HUD code is a separate category, and a panelized or kit package that is assembled and inspected on site is usually reviewed like other site construction. Ask each seller which category their product is in before comparing anything else.

What changes and what does not

Choosing factory-built does not remove the local permit. For factory-built housing with a state insignia of approval, the law treats the unit itself as meeting local construction requirements and limits local plan review of the unit (§ 19981(a)), and the unit may not be modified before installation without state approval (§ 19981(b)). But the local agency still enforces and inspects the installation (§ 19992), and zoning, setbacks, yards, site development, fire zones and architectural review stay local (§ 19993). The state ADU rules—size, height, setbacks, fees and review clocks—apply to the ADU either way.

Decision matrix

1. System and regulatory category

• —Factory-built (HCD-approved): ask for the HCD approval or insignia documentation for the specific model and the installation instructions the local inspector will use.
• —Site-built or panelized: the whole structure is reviewed and inspected locally under the adopted California Building Standards Code.
• —Compare: which agency approves the building itself, and which agency inspects on site.

2. Catalog scope vs site scope

• —Factory-built: a catalog price usually describes the factory unit. Ask exactly where the factory scope ends — at the chassis, the foundation connection, the utility stub-outs or the finished building.
• —Site-built: one contractor may carry the whole building, or design and construction may be split between separate contracts.
• —Compare: a list of every item outside the quoted price.

3. Transport, crane and staging

• —Factory-built: delivery route, street or encroachment permits, crane size and position, overhead lines, trees and neighbor access can decide whether the unit fits the lot at all. Confirm who surveys access and who pays if the plan changes.
• —Site-built: material deliveries and staging still need space, but on a smaller scale.
• —Compare: a written access and lift plan, or a clear statement that one is excluded.

4. Site work

• —Both paths need grading, the foundation, utility trenching and connections, drainage and restoration. These are site conditions, not product features.
• —Compare: whether the foundation design is by the manufacturer, a local engineer or the contractor, and whether soils work is included.

5. Approval and inspection split

• —Factory-built: in-plant approval covers the unit; the local building department inspects the foundation, installation, utility connections and final sign-off (§ 19992).
• —Site-built: local inspections run through each stage of construction.
• —Compare: who schedules inspections and who handles correction notices on each side of the split.

6. Design changes

• —Factory-built: changes to an approved model before installation need state approval (§ 19981(b)); local architectural review can still apply (§ 19993). Ask what options are pre-approved and what triggers a redesign.
• —Site-built: changes go through the design team and the local plan check, with cost and schedule effects written into a change order.

7. Exclusions and allowances

• —List every allowance (finishes, appliances, fixtures) with its amount, and every exclusion (permits and fees, utilities, foundation, crane, restoration).
• —An excluded item still has to be paid for by someone; it is not a saving.

No general cost or speed verdict

This guide does not say one path is cheaper or faster. We have no published dataset that compares all-in costs or durations for factory-built and site-built ADUs in Los Angeles, and outcomes depend on the lot, access, utilities, design and the agencies involved. The useful comparison is between two written proposals for the same property and the same scope.

Documents that let you compare real proposals

1. 01The product or plan set, with its regulatory category and any HCD approval documentation (factory-built) or drawings (site-built).
2. 02A written scope listing what is included, allowances with amounts, and exclusions.
3. 03An access, delivery and staging plan, or a written statement that it is excluded.
4. 04The foundation and utility design responsibility, and whether soils work is included.
5. 05An inspection and closeout responsibility list: who schedules, attends and corrects.
6. 06Contractor license details for everyone doing site work, checked with the CSLB.
7. 07The payment schedule tied to milestones, and how change orders are priced.

To line up a factory-built and a site-built proposal against the same scope rows — transport and crane, foundation, utilities, inspections and restoration — use the private proposal comparison in Project Studio. It totals only amounts you enter and lists the questions each bidder still needs to answer. The cost anatomy guide explains each budget bucket, and the contractor comparison guide covers license and bid checks.