Attached vs. Detached ADU: Which Fits Your Property?
Short answer: an attached ADU (addition or in-law suite) usually costs less because it shares a wall, foundation work and utilities with the house. A detached backyard ADU costs more but gives the most privacy and the best rental setup.
Attached vs. detached comparison
| Feature | Attached ADU | Detached ADU |
|---|---|---|
| Cost | Lower | Higher |
| Privacy | Shares a wall with the main home | Most privacy; a separate home |
| Utilities | Shorter runs from the house | New runs to a separate building |
| Lot size needed | Space for an addition or room within the house | Backyard space with required setbacks |
| Rental appeal | Close to the main household | A more independent rental setup |
| Typical timeline | Usually shorter with shared structure and utilities | Usually longer with a new foundation and utility runs |
Rules in Massachusetts and New Hampshire
MA: one ADU by right up to 900 sq ft or half the main home's floor area, whichever is smaller; attached or detached. See Massachusetts ADU rules.
NH: one attached or detached ADU by right. See our ADU builder in Nashua, NH page.
FAQ
Which is cheaper, an attached or detached ADU?
Usually attached, because it needs less new foundation and shorter utility runs.
Which adds more value?
Detached units are usually easier to rent and feel more like a separate home.
Can I build a detached ADU in Massachusetts?
Yes, one ADU is allowed by right in single-family zones, attached or detached.
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