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 [6]
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) | $275,000-$450,000 |
| Cost per Sq Ft | $400-$600 |
| Garage Conversion | $150,000-$250,000 |
| Prefab / Modular ADU (installed) | $180,000-$350,000 installed (site work, foundations, and buried utilities extra — confirm with manufacturer) |
| City Permit & Plan Review | Confirm with Inspectional Services — building permit fees are valuation-based; budget several thousand dollars on a typical $300K+ build |
| Impact Fees | None for a single ADU — Confirm with Inspectional Services |
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 max floor plate, max massing 24 ft wide x 32 ft deep; state protected-use ADU: up to 900 sq ft or 50% of principal dwelling GFA, whichever is smaller (reasonable local dimensional standards still apply); owner-occupancy: Not allowed — prohibited statewide by the Affordable Homes Act effective Feb 2, 2025).
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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 (Confirm with Inspectional Services — building permit fees are valuation-based; budget several thousand dollars on a typical $300K+ build), plan check (Full architectural drawings plus site plan; sprinkler plan required for most mid-block cottages), and impact fees (None for a single ADU — Confirm with Inspectional Services) are included.
- Permit handling: Confirm they manage Somerville's permit process ("By right: building permit through Inspectional Services (apply online via Citizenserve at somervillema.gov/citizenserve); no special permit, site plan review, or discretionary zoning approval since the 2022 amendment and the 2025 state law. Design standards apply: 3-ft paved walkway sized for an ambulance cot, buried utility services, and automatic sprinklers unless the cottage is on a corner lot, served by a driveway, or abuts an alley", typically Building-permit plan review typically 4-8 weeks — Confirm with Inspectional Services (617-625-6600 ext. 5600, isd@somervillema.gov)) 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 (Building-permit plan review typically 4-8 weeks — Confirm with Inspectional Services (617-625-6600 ext. 5600, isd@somervillema.gov)) 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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