Boston is an outlier in Massachusetts: it zones under its own 1956 enabling act (Chapter 665), so it is exempt from the MA statewide by-right ADU law. Citywide, only interior ADUs are allowed as-of-right — a unit carved within the existing footprint of an owner-occupied 1-, 2-, or 3-family home. Detached and attached (exterior) ADUs require a Zoning Board of Appeal variance everywhere except the Mattapan district, where they are by-right.
Zoning & Size Requirements [1][2][3]
Yes, you can build an ADU in Boston. A detached ADU can be up to the city's size limit under current Massachusetts and Boston zoning rules.
| Allowed Zones | Interior ADUs as-of-right in owner-occupied 1-3 family homes citywide; detached/attached by ZBA variance (by-right only in Mattapan) |
| Max Size (Detached) | Not as-of-right — ZBA variance required (by-right only in Mattapan) |
| Max Size (Attached) | Not as-of-right — ZBA variance required (by-right only in Mattapan) |
| Max Size (JADU) | Interior ADU (the as-of-right type): up to 900 sq ft and no more than 50% of living area, within existing footprint |
| Max Height | 35 feet |
| Max Units | 1 interior ADU per lot (Mattapan may add a detached unit) |
Setback Requirements [1][2][3]
Boston requires a rear setback of about 20 ft for a detached ADU, and no additional parking is required. Owner-occupancy is required.
| Rear Setback | 20 feet (applies to variance/Mattapan detached units) |
| Side Setback | 5 feet (applies to variance/Mattapan detached units) |
| Front Setback | 20 feet (applies to variance/Mattapan detached units) |
| Parking Required | None required for an interior ADU |
| Owner Occupancy | Required (owner-occupied 1-3 family home; condos and LLCs ineligible) |
| Min Lot Size | No minimum for an interior ADU |
Permit Process & Fees [4]
An ADU permit in Boston typically costs $4,500-$7,500 and takes 90-150 days, through a standard permit review.
| Approval Process | Interior ADU: building permit through Inspectional Services (no zoning relief). Detached/attached: building permit + ZBA variance (except Mattapan) |
| Review Timeline | 90-150 days |
| Permit Fees | $4,500-$7,500 |
| Impact Fees | $2,000-$3,500 |
| Plan Check | 4-6 weeks |
| Pre-Approved Plans | Not available |
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Construction Costs [5][6]
Building an ADU in Boston typically costs $200,000-$400,000 ($220-$440 per sq ft); a garage conversion runs $100,000-$220,000.
| Average Build Cost | $200,000-$400,000 |
| Cost per Sq Ft | $220-$440 |
| Garage Conversion | $100,000-$220,000 |
| Prefab ADU (Installed) | $150,000-$320,000 |
Finance Your ADU Project
Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders.
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Rental Income Potential [7][8]
A 2-bedroom ADU in Boston rents for around $2,200-$3,200/mo, an estimated 5-7% annual return.
| Avg Rent (1 Bedroom) | $1,800-$2,400/mo |
| Avg Rent (2 Bedroom) | $2,200-$3,200/mo |
| Short-Term Rental | Not permitted |
| Est. Annual ROI | 5-7% annual return |
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Recent Law Changes
What's Changed Recently
- Boston is exempt from the MA statewide by-right ADU law because it zones under Chapter 665 of the Acts of 1956, not Chapter 40A. Only interior ADUs (within the existing footprint of an owner-occupied 1-, 2-, or 3-family home) are allowed as-of-right citywide
- Detached and attached (exterior) ADUs require a Zoning Board of Appeal variance everywhere except the Mattapan district, where a January 2024 amendment made them by-right
- Boston's Planning Department is advancing a 'Neighborhood Housing' initiative and ADU Guidebook to allow attached/detached ADUs without a variance; draft zoning districts for West Roxbury, Hyde Park, and Roslindale are expected in 2026 but are NOT yet adopted citywide
- The city offers a 0%-interest deferred ADU Loan of up to $50,000 for income-qualified owner-occupants (contact ADU@boston.gov)
- Statewide (excluding Boston), 1,224 ADUs were approved across 217 communities in the first year of the MA Affordable Homes Act by-right mandate (effective Feb 2, 2025)
Tips for Building an ADU in Boston
- Only an interior ADU (basement, attic, or first-floor carve-out) is as-of-right — planning a detached or attached unit means a ZBA variance (unless you're in Mattapan), which adds time and cost
- Owner-occupancy of the 1-3 family home is required; condos and LLCs are not eligible
- Boston's high property values and strong rental market support premium rents
- Historic district restrictions can significantly complicate projects
ADU Builders in Boston
Looking for an ADU builder in Boston? ADU projects need a contractor experienced with local zoning, the permit process, and utility connections — not every general contractor has built one. We match Boston homeowners with experienced local ADU contractors who provide free, no-obligation quotes. Getting at least three bids typically saves 20-40% on the same scope.
Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- Accessory Dwelling Units (ADUs) in Boston — City of Boston (Mayor's Office of Housing) Planning Department — Accessed 2026-06-28
- Enabling Accessory Dwelling Units — Boston Planning Department (citywide ADU zoning) Planning Department — Accessed 2026-06-28
- Frequently Asked ADU Questions — City of Boston (confirms interior-only as-of-right; detached/attached by-right only in Mattapan; exempt from statewide law) Planning Department — Accessed 2026-07-01
- Permitting Your ADU | Boston.gov Planning Department — Accessed 2026-02-24
- Angi — ADU Construction Cost Data Market Data — Accessed 2026-02-24
- Angi — Local ADU Contractor Costs Market Data — Accessed 2026-02-24
- Zillow Rental Manager — Market Rent Data Market Data — Accessed 2026-02-24
- HUD Fair Market Rents Government Data — Accessed 2026-02-24
- ADU FAQ | Boston.gov Planning Department — Accessed 2026-02-24
- Research your ADU Options | Boston.gov Planning Department — Accessed 2026-02-24
- Massachusetts Affordable Homes Act — ADU Provisions Government Data — Accessed 2026-02-24
Frequently Asked Questions
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