An ADU builder in Somerville 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 Somerville'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 Somerville [11][12][13]
Based on our Somerville 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 | Not published online. ISD charges a plan-review fee at application, credited toward the building permit fee if no plan review is needed; the building permit fee itself is set on the Estimated Cost Affidavit — confirm the current rate with ISD before budgeting. |
| Impact Fees | None. No impact or capacity fee appears in the Zoning Ordinance for a single accessory unit, and the state bars any impact fee or study on a Protected Use ADU that is not required for a single-family house (760 CMR 71.03(3)(b)6). Water/sewer connection charges: confirm with the Engineering Division. |
For a full line-item breakdown, see our Somerville ADU cost guide. To confirm what you're allowed to build before you hire, review the Somerville ADU rules (max detached size: Local Backyard Cottage: 576 sq ft floor plate, max 24 ft wide x 32 ft deep, 1.5 stories (SZO §3.1.12.c). State Protected Use ADU: up to 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is smaller (760 CMR 71.02); a detached state ADU may be built to that cap, and city dimensional rules can be no stricter than those for the house or an accessory structure (71.03(3)(b)2). The pending Sept 2026 amendment would swap the 576 sq ft floor plate for a 1,750 sq ft GFA cap.; owner-occupancy: Not required. An owner-occupancy condition on a Protected Use ADU or its principal dwelling is a prohibited regulation statewide (760 CMR 71.03(2)(a)), and the Somerville ordinance imposes none on the Backyard Cottage. (A Council order of Jan 22, 2026, item 26-0100, only asked to discuss an incentive pilot for owner-occupied properties.)).
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How to Vet an ADU Builder in Somerville
- 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 Somerville 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 (Not published online. ISD charges a plan-review fee at application, credited toward the building permit fee if no plan review is needed; the building permit fee itself is set on the Estimated Cost Affidavit — confirm the current rate with ISD before budgeting.), plan check (Full architectural drawings and a site plan showing the 3 ft ambulance-cot walkway, buried utility route and the sprinkler design where required (SZO §3.1.12.f); Fire Prevention Bureau sign-off on the sprinkler exemption for corner, driveway or alley lots.), and impact fees (None. No impact or capacity fee appears in the Zoning Ordinance for a single accessory unit, and the state bars any impact fee or study on a Protected Use ADU that is not required for a single-family house (760 CMR 71.03(3)(b)6). Water/sewer connection charges: confirm with the Engineering Division.) are included.
- Permit handling: Confirm they manage Somerville's permit process ("By right: apply for a building permit with the Inspectional Services Department (ISD) Building Division on the Citizenserve portal (somervillema.gov/citizenserve); no special permit, site plan approval or hearing for a Backyard Cottage or a state Protected Use ADU. An owner living in a 1- or 2-family house may pull the permit; 3-family or larger needs a licensed contractor (CSL, plus HIC for owner-occupied 1-4 unit buildings). Submit an Estimated Cost Affidavit, plans, Workplace Safety and Wage Theft affidavits (contractors also insurance, signed contract, Site Safety Plan, OSHA card). Zoning questions: isdzoning@somervillema.gov; ISD 617-625-6600 x5600, 1 Franey Rd.", typically Not published. ISD posts no review-time target; a plan-review fee is paid when you apply and full plan review precedes issuance — ask ISD (617-625-6600 x5600) for the current queue.) 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 Somerville 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-time target; a plan-review fee is paid when you apply and full plan review precedes issuance — ask ISD (617-625-6600 x5600) for the current queue.) 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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