Prefab ADUs, built in a factory and delivered to the lot, can cost less and go up faster than a cottage built on site, but they do not skip the local work. The foundation, the utility connections, the septic, the permits, and the delivery are all still part of the project. A site-built ADU costs more in some cases and fits more lots.
What prefab covers, and what it does not
A prefab or modular ADU arrives mostly finished: walls, roof, windows, and often the kitchen and bathroom. What stays on site is everything that ties it to the lot:
- The foundation, built before delivery.
- Water, sewer or septic, electrical, and heating connections.
- The septic review under Title 5, and any upgrade it calls for.
- The building permit and the town’s zoning, historic, and conservation reviews.
- Delivery, with room for a truck and often a crane.
- Steps, decks, paths, and landscaping.
What it costs on the Cape
In the Provincetown Independent’s February 2026 report, a Truro homeowner’s ADU from a prefab builder based in Maine cost $270,000, which she said was less than building on site. That figure did not include a new septic system, which they did not need, or the finish work she did herself. The same report put typical Cape ADUs at $225,000 to $500,000.
When site-built makes more sense
- Tight lots and odd shapes, where a delivered box does not fit or cannot be craned in.
- Historic districts, where the design has to match the houses around it.
- Matching the main house: the same shingles, trim, roof pitch, and windows.
- Conversions and additions, where the ADU is part of an existing building.
Questions to ask either way
Who handles the foundation, the utilities, and the septic? What does the price include, and what is extra? How does the unit get to the lot, and over which roads? Who pulls the permits? Clear answers to those questions matter more than the sticker price.