AGrannio

Can You Build an ADU in Boston?

Yes — Boston is governed by Massachusetts's statewide ADU law, which limits what the city can restrict. A 700 sq ft detached ADU here runs about $210,500–$344,500.

Construction in Boston runs about 14% above the Massachusetts average, so budget toward the higher end of these ranges.

Detached ADU (700 sq ft)

$210,500–$344,500

Garage conversion (400 sq ft)

$65,500–$120,500

Statewide protection

Yes

What applies in Boston

Massachusetts's statewide standard (Affordable Homes Act, M.G.L. c.40A §3 (2024)) is the starting point, but parcel-level rules, fees and design review are set locally. Confirm those details with Boston before design.

Estimate your Boston ADU cost

Your property

ADU type

Estimated cost in Massachusetts

$210,500 – $344,500

≈ $301–$492/sq ft turnkey · midpoint $277,500

$188,600
Construction
$50,000
Design & permits
$38,900
Site work
Good news — building an ADU in Massachusetts is very likely allowed.
  • ✓Size within the guaranteed minimum. Your 700 sq ft ADU is at or under the 900 sq ft your city must allow regardless of other rules.
  • ✓No extra parking likely required. Set by local ordinance — commonly 1 space per ADU unless near transit.
  • ✓No owner-occupancy requirement. You can rent out both the main home and the ADU — you do not have to live on the property.
  • ✓Lot has room. A 700 sq ft ADU uses about 12% of your 6,000 sq ft lot — comfortably within typical coverage limits.

Rule basis: Affordable Homes Act, M.G.L. c.40A §3 (2024)

If you rent it out

$1,275–$2,100

est. rent/mo

7.4%

gross yield

14.6 yrs

payback

~$18,972/yr after a 7% vacancy allowance, before financing & maintenance.

Financed over 20 yrs at 7.5%, the build is about $2,236/mo — vs ~$1,700/mo in estimated rent.

Get your detailed feasibility report →

Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.

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Other Massachusetts cities

WorcesterSpringfieldCambridgeLowellSomerville

Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.