Garage Conversion ADUs in Massachusetts & New Hampshire
Short answer: converting an attached or detached garage is often the fastest way to add an ADU, because the foundation, walls and roof already exist. Massachusetts allows one ADU up to 900 sq ft by right in single-family zones (since Feb 2, 2025), and New Hampshire allows one attached or detached ADU by right (since July 1, 2025).
What a garage conversion involves
- Insulating and air-sealing to the energy code, often raising or insulating the slab
- New windows and an egress door
- Plumbing for kitchen and bath, often the biggest cost
- Separate heat/AC (usually a ductless heat pump)
- Fire separation where the garage is attached to the house
What breaks the budget
Long plumbing runs to the sewer or septic, a slab that is too low or cracked, structural changes to the roof, and electrical service upgrades.
Cost
Garage conversions usually cost less than building a new detached ADU because the shell is already there. See our ADU cost guide for current ranges.
FAQ
Do I need a permit to convert a garage into an ADU?
Yes. It needs a building permit, plus plumbing and electrical permits.
Will I lose my parking?
In Massachusetts towns cannot require more than one extra parking space for an ADU, and none within half a mile of transit.
Can a detached garage become an ADU?
Yes, in both states, if it meets setbacks and building code.
Does my septic system matter?
Yes. On septic, adding a bedroom may require a septic review (Title 5 in Massachusetts).
Related services and guides
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