An ADU builder in Cambridge 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 Cambridge'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 Cambridge [12][13][14][15]
Based on our Cambridge 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) | $220,000-$400,000 |
| Cost per Sq Ft | $275-$450 |
| Garage Conversion | $100,000-$250,000 |
| Prefab / Modular ADU (installed) | $250,000-$480,000 |
| City Permit & Plan Review | Building permit $15 per $1,000 of construction cost for buildings of 3 dwelling units or fewer ($20 per $1,000 otherwise), $50 minimum (ISD Building Fees table): about $3,750 on a $250,000 ADU, $5,250 on $350,000. Plan review included; certificate of occupancy $100; trade permits extra. |
| Impact Fees | None. Cambridge has no residential impact fee; its Incentive Zoning housing contribution ($36.36 per sq ft) applies only to non-residential projects over 30,000 sq ft, and inclusionary housing (11.203) only to larger residential projects. 760 CMR 71.03(3)(b)6 bars any ADU-only impact fee. Water and sewer connection charges from DPW and the Water Department may apply to a detached unit; confirm before design. |
For a full line-item breakdown, see our Cambridge ADU cost guide. To confirm what you're allowed to build before you hire, review the Cambridge ADU rules (max detached size: No local size cap since Feb 10, 2025 (Ord. 2025-1 struck the 900 sq ft / 35% rule in old 4.22.1). To keep state Protected Use status the unit must be 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is less; a larger unit is simply an additional dwelling unit, which Cambridge also allows as of right with no FAR limit for residences (5.25.4(a)) and no lot-area-per-unit rule.; owner-occupancy: Not required: 760 CMR 71.03(2)(a) prohibits an owner-occupancy requirement for either unit, and Cambridge's 4.22 imposes none. The Article 2 definition (Ord. #1408, 2019) keeps the accessory apartment under the same ownership as the main house, i.e. it cannot be sold off separately.).
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How to Vet an ADU Builder in Cambridge
- 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 Cambridge 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 (Building permit $15 per $1,000 of construction cost for buildings of 3 dwelling units or fewer ($20 per $1,000 otherwise), $50 minimum (ISD Building Fees table): about $3,750 on a $250,000 ADU, $5,250 on $350,000. Plan review included; certificate of occupancy $100; trade permits extra.), plan check (Included in the building permit fee per the ISD Building Fees table; there is no separate ADU plan-check queue.), and impact fees (None. Cambridge has no residential impact fee; its Incentive Zoning housing contribution ($36.36 per sq ft) applies only to non-residential projects over 30,000 sq ft, and inclusionary housing (11.203) only to larger residential projects. 760 CMR 71.03(3)(b)6 bars any ADU-only impact fee. Water and sewer connection charges from DPW and the Water Department may apply to a detached unit; confirm before design.) are included.
- Permit handling: Confirm they manage Cambridge's permit process ("Building permit only, filed through ISD's OpenGov portal (cambridgema.portal.opengov.com); no BZA hearing since Ord. 2025-1 (Feb 10, 2025) deleted the 4.22 special-permit process. Steps: 1) confirm district and yards on the 2025 zoning map; 2) Cambridge Historical Commission sign-off if the property is in a historic or conservation district, is a landmark, or a building 50+ years old is to be demolished; 3) City Council curb-cut approval if a new driveway is needed; 4) submit plans with the estimated-cost affidavit and a homeowner's or licensed construction supervisor's affidavit; 5) separate electrical, plumbing and gas permits; 6) inspections, then a certificate of occupancy ($100 for the first unit, $50 per additional unit).", typically Not published. ISD posts no review target for building permits; the state ADU law makes the unit as-of-right so there is no hearing, but Historical Commission review in a historic or conservation district and a curb-cut vote add weeks.) 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 Cambridge 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 (Not published. ISD posts no review target for building permits; the state ADU law makes the unit as-of-right so there is no hearing, but Historical Commission review in a historic or conservation district and a curb-cut vote add weeks.) 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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