The permission comes first
An accessory dwelling unit is a second, smaller dwelling on a lot that already has a house on it. Whether you may build one, how large it may be, how far it must sit from the boundary, whether it needs its own parking, and whether it can be metered or sold separately are all questions of local land use law. None of them is a question about the building.
This inverts the usual order. On a primary house, you design and then confirm compliance. On an ADU, the envelope of what is permitted is often so tight that it determines the design outright. Establish the rules first and the unit becomes a straightforward object. Do it the other way round and you will be redesigning a building you have already bought.
Why prefabrication suits this case unusually well
Almost every property that makes offsite construction attractive is present in an ADU. The unit is small, so it fits on a truck without the width permits a full-size module needs. The design repeats, so a manufacturer can amortise the drawings across many units. The site is a back garden, which is a terrible place to run a construction site for months and a perfectly good place to set a finished building in a day.
The last of those is the one homeowners feel. An ADU is built on a lot that somebody is living on. Every week of site work is a week of that. Compressing it is worth real money to the person whose garden it is, independent of what it does to the construction cost.
What still happens on your land
A delivered unit is not a finished project, and the remainder is the part that varies by site.
The foundation or piers, designed for your soil. The connections: water, sewer or septic, power, and any gas. The distance from the existing house to the point of connection, which is the single largest swing factor in an ADU budget and is a function of your lot rather than your unit. Access for the truck and the crane, which on an infill lot may mean over the house, and that decides crane size. Site restoration afterwards. And permits, inspections and fees, all local.
A quotation that covers the unit is quoting perhaps half the project. That is not a complaint about manufacturers, it is the same scope boundary described under modular costs, and it is the reason the budgeting method on the front sheet starts with the site rather than the building.
California, as the case study rather than the rule
California has legislated on ADUs more aggressively than any other state, which makes it the most documented case and the one most often cited elsewhere. The state's Department of Housing and Community Development publishes guidance and an ADU handbook, maintains a technical assistance and enforcement function for local ordinances, and notes that the Government Code sections relevant to state ADU and junior ADU law were renumbered following Senate Bill 477 in 2024. [1]
Two things follow, and the second is the one people miss. First, in California the state has removed a great deal of local discretion, so what your city may require is constrained by state law. Second, that renumbering is a warning about the shelf life of any citation in this area. A page quoting a Government Code section without saying which version it read is quoting something that may have moved.
Everywhere outside California, none of this transfers. State ADU law ranges from comprehensive to nonexistent, and where it is absent the local ordinance is the whole answer.
Note
ADU rules are set locally and change frequently. Nothing on this page establishes what is permitted on your lot. The method in the next section does.
What a manufacturer can honestly promise
An ADU manufacturer can commit to a production time and a delivered specification. Those are inside their control and a good one will put dates against them.
What no manufacturer can promise is a move-in date, because the two longest items on an ADU programme are the planning approval and the utility connection, and both belong to organisations the manufacturer has no relationship with. A quoted total timeline that includes permitting is a guess dressed as a schedule, and it is the single most reliable indicator of how a supplier will handle the rest of the job.
The useful question is narrower and it gets a real answer: how long after you release the design does the unit arrive, and what has to be true on the site by then. Everything before that is yours to manage.
How to establish the rules for your own lot
Four steps, in this order, before you speak to a manufacturer.
Find your zoning district and the ordinance text that governs accessory dwellings in it, from the jurisdiction's own published code rather than from a summary. Read the dimensional standards: maximum floor area, height, setbacks, lot coverage, and separation from the primary dwelling. Establish whether state law overrides any of it, which in some states it does and in most it does not. Then ask the planning department one written question: on this specific parcel, what is the largest accessory dwelling that could be approved, and by what process.
That written answer is the design brief. Take it to a manufacturer and the conversation becomes a procurement exercise. Skip it and the conversation is a sales pitch against constraints neither of you has read.
Questions
- Is a prefab ADU cheaper than a site built one?
- Often, and for a reason that has little to do with construction: the site disruption is shorter, and on an occupied lot that has value. The unit price alone rarely settles it, because the connection distances and access on your particular lot can swing the total either way.
- Does an ADU need its own utility connections?
- That is a local question and it is worth asking early, because separate metering is expensive and is sometimes required, sometimes forbidden, and sometimes optional. The utility and the planning department will each answer a different half of it.
Sources
- 1California Department of Housing and Community Development. Accessory Dwelling Units. Accessed 2026-08-29
- 2U.S. Census Bureau. Survey of Construction: Definitions. Accessed 2026-08-29