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Can You Build an ADU in Somerville?

Yes — Somerville is governed by Massachusetts's statewide ADU law, which limits what the city can restrict. A 700 sq ft detached ADU here runs about $203,500–$332,500.

Construction in Somerville runs about 10% above the Massachusetts average, so budget toward the higher end of these ranges.

Detached ADU (700 sq ft)

$203,500–$332,500

Garage conversion (400 sq ft)

$63,500–$116,000

Statewide protection

Yes

What applies in Somerville

Massachusetts' 2024 Affordable Homes Act allows ADUs up to 900 sq ft (or half the size of the primary home) by right in single-family zones statewide, without an owner-occupancy requirement.

Because Massachusetts has a statewide ADU law (Affordable Homes Act, M.G.L. c.40A §3 (2024)), Somerville must follow these minimums even where its older zoning code is stricter. Confirm fees and design standards with the Somerville planning department.

Estimate your Somerville ADU cost

Your property

ADU type

Estimated cost in Massachusetts

$203,500 – $332,500

≈ $290–$475/sq ft turnkey · midpoint $268,000

$182,300
Construction
$48,200
Design & permits
$37,500
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

7.6%

gross yield

14.1 yrs

payback

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

Financed over 20 yrs at 7.5%, the build is about $2,159/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

BostonWorcesterSpringfieldCambridgeLowell

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.