ADU Builders in Boston, MA (2026)

Not by right — Boston allows ADUs only case by case. An interior ADU (a basement, attic or first-floor unit built entirely within the existing footprint of an owner-occupied 1-, 2- or 3-family home) is by right citywide and needs only an Inspectional Services long-form building permit. A detached or attached ADU is by right only in Mattapan's R1 and R2 subdistricts (Article 60, adopted January 2024); everywhere else it needs a Zoning Board of Appeal variance ($150 filing fee), because the code has no rule that allows it. Read the current Boston ADU rules before hiring anyone; a contractor quote here is for planning only.

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 ^[10]^[11]

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)	$75,000-$350,000
Cost per Sq Ft	$150-$400
Garage Conversion	$150,000-$275,000
Prefab / Modular ADU (installed)	$180,000-$320,000
City Permit & Plan Review	Long-form building permit $50 plus $10 per $1,000 of estimated construction cost (ISD fee schedule dated May 15, 2023): about $950 on a $90,000 interior ADU, $3,050 on a $300,000 detached ADU; ZBA appeal $150 for a 1-3 family home.
Impact Fees	None: ISD's fee schedule lists no impact or capacity fee. Trade permits are small (electrical $20 plus $1 per fixture, plumbing $20 plus $5 per fixture, gas $20 plus $5 per appliance). Ask BWSC about connection charges if a detached ADU needs its own water or sewer service.

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: By right only in Mattapan R1/R2: building floor plate up to 900 sq ft or the floor plate of the main house, whichever is less (Article 60). Elsewhere the code has no detached-ADU rule, so any size needs a ZBA variance.; owner-occupancy: Required: the owner must live on the parcel, which must be a 1-, 2- or 3-family home. Condominiums and LLC-owned properties are ineligible. The state ban on ADU owner-occupancy rules (G.L. c. 40A s. 3) does not reach Boston.).

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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 (Long-form building permit $50 plus $10 per $1,000 of estimated construction cost (ISD fee schedule dated May 15, 2023): about $950 on a $90,000 interior ADU, $3,050 on a $300,000 detached ADU; ZBA appeal $150 for a 1-3 family home.), plan check (About 5 weeks for a complete long-form application (city ADU FAQ); longer if plans are revised, documents are missing or zoning relief is needed.), and impact fees (None: ISD's fee schedule lists no impact or capacity fee. Trade permits are small (electrical $20 plus $1 per fixture, plumbing $20 plus $5 per fixture, gas $20 plus $5 per appliance). Ask BWSC about connection charges if a detached ADU needs its own water or sewer service.) are included.
Permit handling: Confirm they manage Boston's permit process ("Every ADU needs an ISD long-form building permit (online or at 1010 Massachusetts Ave) with architect-stamped plans, the contractor agreement and workers' comp certificate, plus electrical, plumbing, gas or sprinkler permits as needed. Interior ADUs and Mattapan R1/R2 exterior ADUs need no zoning relief. Other detached or attached ADUs get an ISD zoning refusal letter, then a ZBA appeal filed within 45 days ($150) and a public hearing. Register the finished unit with the city's annual rental registry.", typically ISD plan review takes about 5 weeks for a complete application (city ADU FAQ); a ZBA variance adds a hearing cycle. City guidance: design up to 3 months, permitting up to 3 months, construction 6-12 months.) 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 (ISD plan review takes about 5 weeks for a complete application (city ADU FAQ); a ZBA variance adds a hearing cycle. City guidance: design up to 3 months, permitting up to 3 months, construction 6-12 months.) 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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Frequently Asked Questions

How much do ADU builders charge in Boston?

ADU builders in Boston typically quote $75,000-$350,000 for a new detached unit, or about $150-$400 per square foot, including design, permitting, and construction. A garage conversion runs $150,000-$275,000 and a prefab/modular ADU averages $180,000-$320,000. Add city permit and plan-review fees (Long-form building permit $50 plus $10 per $1,000 of estimated construction cost (ISD fee schedule dated May 15, 2023): about $950 on a $90,000 interior ADU, $3,050 on a $300,000 detached ADU; ZBA appeal $150 for a 1-3 family home.) plus impact fees of None: ISD's fee schedule lists no impact or capacity fee. Trade permits are small (electrical $20 plus $1 per fixture, plumbing $20 plus $5 per fixture, gas $20 plus $5 per appliance). Ask BWSC about connection charges if a detached ADU needs its own water or sewer service.. Quotes for the same scope commonly vary 20-40% between builders, so collect at least three bids.

How do I find a good ADU builder in Boston?

Look for a licensed contractor with completed ADU projects in Boston specifically — ADUs involve local zoning, setback rules, and utility connections that general remodelers may not have handled. Verify the license, ask for an ADU-specific portfolio and references, confirm they carry liability and workers' comp insurance, and get a written fixed-price or detailed bid. Submit a request below and we'll forward it to local ADU builders serving Boston for free quotes.

Do ADU builders in Boston handle permits?

Most full-service ADU builders manage the permit process for you. In Boston the approval process is "Every ADU needs an ISD long-form building permit (online or at 1010 Massachusetts Ave) with architect-stamped plans, the contractor agreement and workers' comp certificate, plus electrical, plumbing, gas or sprinkler permits as needed. Interior ADUs and Mattapan R1/R2 exterior ADUs need no zoning relief. Other detached or attached ADUs get an ISD zoning refusal letter, then a ZBA appeal filed within 45 days ($150) and a public hearing. Register the finished unit with the city's annual rental registry." with a typical review timeline of ISD plan review takes about 5 weeks for a complete application (city ADU FAQ); a ZBA variance adds a hearing cycle. City guidance: design up to 3 months, permitting up to 3 months, construction 6-12 months.. Confirm in writing whether permit fees, plan check (About 5 weeks for a complete long-form application (city ADU FAQ); longer if plans are revised, documents are missing or zoning relief is needed.), and impact fees are included in the bid or billed separately.

How long does it take to build an ADU in Boston?

Plan for the permit review (ISD plan review takes about 5 weeks for a complete application (city ADU FAQ); a ZBA variance adds a hearing cycle. City guidance: design up to 3 months, permitting up to 3 months, construction 6-12 months. in Boston) plus construction. Most detached ADUs take 4-8 months of construction after permits are issued; garage conversions and prefab units are often faster. Ask each builder for a realistic schedule and what could delay it.

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Build Cost	$75,000-$350,000
Garage Conv.	$150,000-$275,000
Permit Fees	Long-form building permit $50 plus $10 per $1,000 of estimated construction cost (ISD fee schedule dated May 15, 2023): about $950 on a $90,000 interior ADU, $3,050 on a $300,000 detached ADU; ZBA appeal $150 for a 1-3 family home.
Timeline	ISD plan review takes about 5 weeks for a complete application (city ADU FAQ); a ZBA variance adds a hearing cycle. City guidance: design up to 3 months, permitting up to 3 months, construction 6-12 months.
1BR Rent	$2,500-$2,900/mo

What ADU Builders Cost in Boston [10][11]