A prefab ADU is built in a factory. What it costs is decided on your lot.

We do not publish a prefab price, because we do not sell prefab, and a number we had read somewhere for a manufacturer we do not contract with is not a number anyone should plan against. What we can tell you is how the figure is built. The price on a factory catalog is the unit leaving the factory; the delivered price adds site work and access, an engineered foundation poured on your lot, transport and the crane, utility runs including the panel upgrade a second dwelling often forces, the permit path with its city and district fees, and the trades that connect everything after the unit is set. Ask any manufacturer for that list in writing, with the exclusions named. For the alternative, Avorino publishes a detached ADU at typically $245K – $440K+ for 500 – 1,200 sqft, and the cost estimator prices it for your own city and finish level.

Sometimes, and the honest answer is that it depends on the lot rather than on the method. Prefab is at its strongest on a flat, open, rectangular lot where a catalog plan already clears the setbacks and a crane can reach the back: there the factory is doing work efficiently that a site crew would do slowly. On a sloped, narrow, landscaped or view-sensitive lot, the site work, the access problem and the design compromises eat the advantage, and the two can land close together. Compare delivered price against delivered price, never a factory price against a full contract. Avorino's own published Orange County ranges are $245K – $440K+ for a detached ADU and $85K – $170K+ for a garage conversion, both covering design through final inspection. City permit, plan-check and inspection fees are paid by the owner to the city and are not in either range.

Six things, in the order they usually appear on the final bill: site work and access, the engineered foundation, delivery and the crane, the utility runs, the permit path with its fees, and the trades that finish the job once the unit is on the ground. None of these are optional, and a catalog price typically covers none of them; an installed or turnkey quote may cover some, so read the exclusions. This is why two prefab quotes for the same unit on two different lots can be far apart while the catalog price is identical. The impact-fee exemption is one of the few line items state law settles for you: §66311.5(c)(1) bars the fee entirely on a unit of 750 square feet of interior livable space or less.

The factory portion is typically fast and predictable, which is prefab's best argument, but it runs alongside a local schedule that does not change: design and engineering, the city's review, then the foundation and utility work that has to be finished before anything can be set. §66317(a)(3) gives the city 60 days from a completed application, after which the application is deemed approved, and the clock starts at completeness rather than at first submission, so correction rounds decide when it begins. Avorino builds a detached unit in typically 5–8+ mo of construction, with design, engineering and that same city review ahead of it. Neither route skips the permit.

Yes, and it is the same permit. State ADU law is written about the unit rather than about how the unit was assembled, so a factory-built ADU is reviewed ministerially under Government Code §66317(a)(1), on the 60-day clock in §66317(a)(3), with the setback standards in §66314 applying as they would to anything else. The city still permits and inspects the foundation, the utility connections and the setting of the unit. Prefab buys you factory schedule certainty; it does not buy you a different approval path.

Answer three questions and the lot usually decides it for you. Can a truck and a crane reach the spot the unit has to sit, past the gates, the trees and the overhead lines? Does a catalog plan fit inside your setbacks and lot coverage without a redesign? And are you content with a catalog elevation rather than one drawn against the house in front of it? Three yeses point at prefab, and the schedule certainty is real. A no on any of them points at a site-built unit, because that is the case where the design has to answer the lot rather than the other way round.

No. Avorino designs, permits and builds custom, site-built ADUs across 37 Orange County cities under California General-B License #1107538, and we do not sell or install factory units. That is also why this page carries no prefab pricing: we publish figures for work we do, and the ones we publish are $245K – $440K+ for a detached ADU and $85K – $170K+ for a garage conversion, design through final inspection on one fixed-price contract. If prefab is the right answer for your lot, the comparison above is written to help you buy one properly rather than to talk you out of it.