A prefab ADU is built in a factory and delivered to your lot; a custom (stick-built) ADU is built on site. Prefab’s advertised price usually covers the box — not the foundation, utility connections, craning, permits or site work that finish the job. Once those are added, the totals often land closer than the brochures suggest. The honest rule: flat, open lots with easy access give prefab its best case; sloped lots, tight side yards, garage conversions and design-sensitive neighborhoods favor custom.
We build custom ADUs for a living, so read this knowing where we stand — but the comparison below is the same one we give homeowners who ask us honestly whether they should just order a prefab. Sometimes the answer is yes. Here is how to think about it.
01What “prefab” actually covers — three different things
- Modular ADU
Built as complete room-sized modules in a factory to the same residential building code as a site-built home, trucked in and craned onto a prepared foundation. The most house-like prefab.
- Manufactured ADU
Built to the federal HUD code (the mobile-home standard) rather than the local residential code, on a permanent chassis. Usually the cheapest sticker price; financing, appraisal and resale behave differently.
- Panelized ADU
Factory-built wall and roof panels, assembled on site. A hybrid — faster framing, but most of the project (foundation, utilities, finishes) still happens on your lot like a custom build.
“Stick-built” — what we do — means the unit is framed and finished on site, designed for your specific lot from the first sketch.
02The price-tag illusion
Prefab marketing quotes the unit; your budget has to cover the project. Whichever route you choose, someone still has to pay for:
- Foundation — engineered, permitted and poured before anything arrives on a truck.
- Utility connections — sewer, water, electrical from the main house or street to the unit; panel upgrades where needed.
- Delivery and craning — a crane needs somewhere to park, and overhead wires or mature trees can complicate or kill the lift.
- Permits, plan check and city fees — the same review as any dwelling; see our cost guide for the OC fee picture.
- Site work — grading, drainage, walkways, landscaping repair after heavy equipment.
That is why we tell homeowners to compare all-in totals, not brochures. For reference, our published custom ranges: garage conversions $90K–$150K, detached custom builds $200K–$450K+ — those figures include design, permits and construction.
03Where prefab genuinely wins — and where it can’t play
| Your situation | Better fit | Why |
|---|---|---|
| Flat lot, wide access, standard layout is fine | Prefab has a real case | Factory speed pays off when the site asks no questions |
| Garage conversion | Custom only | You can’t deliver a factory box into an existing garage |
| Sloped or hillside lot | Custom | Stepped foundations and grading demand per-lot engineering |
| Tight side-yard access, overhead wires, mature trees | Custom | If the crane can’t reach, prefab can’t land |
| HOA review or historic district (Irvine, Mission Viejo, Old Towne Orange) | Custom | Materials and massing must match — catalog boxes struggle in design review |
| You want the unit to match the main house | Custom | Rooflines, stucco, windows designed to belong, not just to fit |
| Maximizing a specific odd-shaped yard | Custom | An L-shaped or narrow design squeezes value a standard box leaves on the table |
Orange County adds its own wrinkle: many of the lots we build on are exactly the hard cases — postwar neighborhoods with narrow side yards, hillside cities like Mission Viejo, HOA communities like Irvine, and historic districts in Orange. That is not prefab country.
04The timeline truth
Factories build fast — but the factory is rarely the bottleneck. Permits, foundation work and utility connections run on the same calendar either way, and a garage conversion we build runs about 4–5 months including permits; detached custom builds 6–12 months. Prefab can compress the framing weeks; it cannot compress plan check. The full phase-by-phase schedule is in our timeline guide.
05How to decide in one visit
The decision is almost always made by the lot, not the catalog: slope, access width, sewer distance, panel capacity, and what your city or HOA will approve. Those are exactly the conditions we measure in a free on-site assessment — and if your lot is genuinely a flat-and-easy prefab case, we’ll tell you that too. A specialist who only wins when you pick their product isn’t advising you; we’d rather be the second opinion that saves you a six-figure mistake.