An ADU builder in Boston is a licensed contractor who designs and constructs accessory dwelling units — detached backyard units, garage conversions, attached additions, and junior ADUs (JADUs). Because an ADU is a complete second dwelling, the builder coordinates foundation, framing, plumbing, electrical, and HVAC work and pulls it through Boston's permit process and utility connections. Not every general contractor has built one, so the right builder is one with ADU-specific experience in Massachusetts.
What ADU Builders Cost in Boston [5][6]
Based on our Boston cost data, expect the following ranges from local ADU builders. Quotes for the same scope typically vary 20-40% between contractors, so getting at least three bids is the single best way to control cost.
| New Detached ADU (all-in) | $200,000-$400,000 |
| Cost per Sq Ft | $220-$440 |
| Garage Conversion | $100,000-$220,000 |
| Prefab / Modular ADU (installed) | $150,000-$320,000 |
| City Permit & Plan Review | $4,500-$7,500 |
| Impact Fees | $2,000-$3,500 |
For a full line-item breakdown, see our Boston ADU cost guide. To confirm what you're allowed to build before you hire, review the Boston ADU rules (max detached size: Not as-of-right — ZBA variance required (by-right only in Mattapan); owner-occupancy: Required (owner-occupied 1-3 family home; condos and LLCs ineligible)).
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How to Vet an ADU Builder in Boston
- License: Confirm a current contractor's license in Massachusetts and that it's in good standing. Ask for the license number and verify it with the state board.
- ADU-specific experience: Ask how many ADUs they've completed in or near Boston and request addresses or photos. ADU permitting and setbacks differ from a kitchen remodel.
- Portfolio & references: Get references from recent ADU clients and, if possible, visit a finished unit.
- Insurance: Verify general liability and workers' compensation coverage. Ask for a certificate of insurance.
- Written, detailed bid: Insist on an itemized, fixed-price or clearly-scoped bid — not a verbal estimate. It should state whether permits ($4,500-$7,500), plan check (4-6 weeks), and impact fees ($2,000-$3,500) are included.
- Permit handling: Confirm they manage Boston's permit process ("Interior ADU: building permit through Inspectional Services (no zoning relief). Detached/attached: building permit + ZBA variance (except Mattapan)", typically 90-150 days) and who is responsible if revisions are required.
- Payment schedule: Avoid large upfront payments. A reasonable schedule ties payments to completed milestones.
Questions to Ask Before You Hire
- How many ADUs have you built in Boston in the last two years?
- Is your bid fixed-price, and what would trigger a change order?
- Are permits, plan check, and utility connection fees included?
- What's a realistic timeline, including permit review (90-150 days) and construction?
- Who is my point of contact and how are delays communicated?
- What warranty do you provide on workmanship?
- Can you share three recent Massachusetts ADU client references?
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Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders.
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