How Much Does It Cost to Build an ADU in Massachusetts? (2026 Guide)

The honest answer to "what does an ADU cost in Massachusetts?" is: it depends enormously on what you're starting with. A basement that already has ceiling height and egress is a completely different project from a backyard cottage that needs a foundation, utility trenching, and its own roof. Here's how 2026 pricing actually breaks down in Greater Boston.

Cost by ADU Type (Greater Boston, 2026)

• Basement ADU: $80K–$150K. The structure exists — you're paying for egress, insulation, a kitchen and bath, and fire separation. See our basement finishing guide for what's involved structurally.
• Garage conversion: $120K–$220K. Slab insulation, new wall assemblies, utilities from the main house. Detached garages cost more than attached ones because of trenching.
• Attached addition ADU: $180K–$350K. New construction tied into your existing structure, sharing one wall and often utility runs.
• Detached backyard cottage: $250K–$500K+. Full new construction: foundation, framing, roof, independent systems. Figure $275–$450 per square foot depending on finishes.

The Costs Homeowners Miss

• Utility connections: $10K–$25K for water, sewer, and electric trenching to a detached structure — the single biggest variable on cottage builds.
• Septic upgrades: $25K–$40K if your Title 5 system can't support the added bedrooms.
• Site work: ledge, grading, tree removal, and drainage can add five figures on tight or sloped lots.
• Design and permitting: plans, structural engineering, and fees typically run $8K–$20K.

Is It Worth It?

A one-bedroom ADU in Greater Boston rents for $2,000–$3,000 a month — our ADU rental income breakdown runs the full payback math. Between rental income and the appraisal bump, a well-built ADU is one of the few six-figure projects that pays you back monthly.

Want a real number instead of a range? Our ADU design-build team provides fixed-price proposals after a free feasibility visit.