Build costs, cost per square foot, garage conversions and rents across 8 Massachusetts cities, from each city’s own cost guide.
Building an ADU in Massachusetts costs $75,000–$450,000 all-in across the 8 cities we cover (2026), or about $150–$450 per square foot. The middle of the 8 city ranges below sits around $310,000. Converting an existing garage runs $100,000–$275,000.
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| City | Typical build cost | Cost per sq ft | Garage conversion | 2-BR rent |
|---|---|---|---|---|
| Boston | $75,000-$350,000 | $150-$400 | $150,000-$275,000 | $3,000-$3,500/mo |
| Cambridge | $220,000-$400,000 | $275-$450 | $100,000-$250,000 | $3,800-$4,300/mo |
| Medford | $220,000-$400,000 | $275-$450 | $100,000-$250,000 | $3,000-$3,350/mo |
| Newton | $250,000-$450,000 | $275-$450 | $100,000-$250,000 | $3,100-$4,300/mo |
| Quincy | $220,000-$400,000 | $275-$450 | $100,000-$250,000 | $2,650-$3,150/mo |
| Somerville | $220,000-$400,000 | $275-$450 | $100,000-$250,000 | $3,450-$3,750/mo |
| Waltham | $200,000-$400,000 | $275-$450 | $100,000-$250,000 | $3,050-$3,640/mo |
| Worcester | $220,000-$360,000 | $275-$450 | $100,000-$250,000 | $1,850-$2,250/mo |
Each figure is the city’s own cost guide; click a city for what is included, permit fees and the sources behind it.
Across the 8 Massachusetts cities we cover, building a new ADU runs $75,000 to $450,000 all-in, depending on the city, size and finish. The middle of the city ranges sits around $310,000. Boston has the lowest published range ($75,000-$350,000) and Newton the highest ($250,000-$450,000).
$150 to $450 per square foot across the 8 Massachusetts cities we cover. Smaller units cost more per square foot, because the kitchen and bathroom are the expensive rooms.
Usually. Converting an existing garage runs $100,000 to $275,000 in the Massachusetts cities we cover, against $75,000 to $450,000 for a new detached unit, because the foundation, walls and roof already exist.
A two-bedroom unit rents for about $1,850 to $4,300 a month across these cities. Each city's cost page shows its own rent figure and source.
Yes. An ADU is a home, so it needs a building permit, and the zoning rules, fees and review times are set city by city. Our Massachusetts ADU laws page compares them.
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