Prefab ADUs: Factory-Built Units, Honestly Assessed

"Why is site-built construction so slow and expensive? Just buy a prefab!" — every ADU forum, daily. Prefab is a real and growing option with genuine strengths. It also has a brochure-price problem that this lesson exists to correct.

What prefab means here: substantial factory construction — full volumetric modules trucked in and craned onto a foundation, or heavily panelized kits assembled on-site in days. (Lesson 13.9 covers the modular/manufactured distinctions and their different approval paths.)

The genuine strengths: factory conditions — indoor assembly, repeatable QC, no rain delays; schedule compression — the unit is built while your site work and permits proceed in parallel, and crane day is genuinely thrilling (a house arrives before lunch); price certainty on the box — the factory quote for the unit itself is firm in a way site-bids aren't; and design maturity — established makers have refined their handful of floor plans through hundreds of iterations (they've internalized most of Module 8 so you don't have to).

The brochure-price problem — read this twice: the quoted price is the box. Your project still needs: the foundation (built on-site, to the maker's spec — 12.3 doesn't disappear), all Module 7: utility trenching, connections, panel work, sewer; site work: access, grading, that crane (crane day costs real money — $5K–$20K+ depending on reach; a unit craned over your house is the expensive kind); permits — the full Module 10 process for the site, foundation, and utilities (factory certification streamlines the unit's approval, not the site's); plus delivery, and finish items the spec excludes (steps, decks, landscaping — 6.8's list lives here too). All-in, prefab typically lands within 10–25% of comparable site-built cost — sometimes better, occasionally worse (tight-access lots punish prefab hard: if the crane can't reach, the deal's off). The savings are real but modest; the certainty and speed are the honest headline benefits.

The fit test: prefab shines with good site access (crane reach, truck route), a standard flat lot, happiness with the maker's existing plans (customization erodes every advantage), and a premium on schedule. Site-built wins on tight access, slopes, custom design, conversions (obviously), and when a strong local builder's price is competitive anyway.

Vet a prefab maker like a contractor (Module 11 applies!): completed installs you can visit, all-in reference pricing from real projects (not the brochure), who handles the site scope and permits (some makers are turnkey; many leave you to GC the site side — which is 11.11 wearing a hard hat), and their units' approval status with your city.