Prefab ADU vs. Site-Built ADU in Massachusetts
Short answer: a prefab or modular ADU is built in a factory and set on your lot; a site-built ADU is framed on your property. Prefab can shorten the build, but you still pay for the foundation, utilities, permits, delivery and crane, so total cost is often closer to site-built than the base price suggests. FJ Global Custom Builds builds site-built ADUs.
Prefab vs. site-built comparison
| Feature | Prefab or modular ADU | Site-built ADU |
|---|---|---|
| Design flexibility | Choose from factory layouts and available options | Custom layouts to match the main house and property |
| Timeline | Factory construction can shorten the build; site work is still needed | Framed and completed on your property |
| Hidden costs | Foundation, utilities, crane and delivery add to the unit price | Include foundation and utilities in the full project estimate |
| Fit on tight or sloped lots | Delivery trucks and crane access can limit the fit | Can be planned around tight access or a sloped lot |
| Permits | Full permits required in Massachusetts | Full permits required in Massachusetts |
When site-built makes more sense
- Tight access for delivery trucks
- Attached ADUs and additions
- Garage conversions
- Custom layouts to match the main house
FAQ
Are prefab ADUs allowed in Massachusetts?
Yes. They follow the same ADU rules and need a building permit.
Is a prefab ADU cheaper?
The unit price can be lower, but site work, foundation, utilities and delivery add a lot. Compare full installed prices.
Do you install prefab ADUs?
We build site-built ADUs. We can give you a site-built price to compare with a prefab quote.
Related services and guides
Planning an ADU? Get a free estimate from a licensed MA contractor with $1M insurance. Call (781) 527-9445 or fill out the form.
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