ADU Rules in Boston, MA (2026)
ADU rules, permit requirements, size limits, and fees for Boston, MA. Source-cited from official municipal code.
Whether ADUs are allowed in Boston, MA has not been confirmed — see the official source before proceeding.
Whether Boston, MA permits accessory dwelling units (ADUs) has not been confirmed from an official source. ADUs — also called granny flats, in-law units, or secondary suites — are secondary housing units on a single-family or multifamily lot. Contact the local building department at (617) 635-5300 to verify current ADU rules before proceeding, as regulations may have changed since this data was last reviewed.
ADU Size Limits
ADU Size Limits in Boston: Detached ADU: size varies by zoning — see official source (max height: varies — see official source). Attached ADU: size varies by zoning — see official source.
| Type | Max Size | Max Height |
|---|---|---|
| Detached ADU | See official source | See official source |
| Attached ADU | See official source | See official source |
Permit Fees
ADU Permit Fees in Boston: Plan check fee: Varies. Building permit fee: Varies. Impact fees: None. School fees: None or N/A. Total estimated permit cost: Varies.
| Fee Type | Amount |
|---|---|
| Plan Check | $500–$3,000 |
| Building Permit | Based on construction value — typically $2,000–$8,000 |
| Impact Fees | Massachusetts municipalities may not charge impact fees beyond standard connection fees; Boston does not have a general impact fee system. Utility connection fees apply. |
| School Fees | No school impact fees in Massachusetts — school funding is through property tax and state aid |
| Total Estimate | See official source |
Permit Timeline
Boston ADU Permit Timeline: Permit timeline has not been confirmed — contact the department for current processing times. Apply through the local building department online permit portal. Phone: (617) 635-5300.
Setback Requirements
ADU Setback Requirements in Boston: Front setback: follows zone standard. Side setback: no minimum. Rear setback: no minimum.
Parking Requirements
Parking Requirements: Parking requirements have not been confirmed — see official source.
Restrictions & Requirements
- ADU must comply with Boston Zoning Code dimensional standards for the zone
- Detached ADU must be located in rear yard — not in front yard setback
- ADU cannot be sold separately from the principal lot
- Boston Short-Term Rental regulations strictly limit STR — ADU as primary Airbnb rental is restricted
- Historic district requirements (Back Bay, Beacon Hill, South End) may restrict exterior modifications
- ADU must comply with Boston's energy efficiency requirements (Stretch Energy Code)
- ADU must have permanent foundation — no recreational vehicles or mobile homes
How Many ADUs Are Allowed?
Number of ADUs Allowed in Boston: Single-family lots: not confirmed — see official source. Multifamily properties: not confirmed — see official source.
- Single-Family Lots
- 1 ADU per single-family lot by-right per Massachusetts Affordable Homes Act (2024); additional units may be possible through Boston's ADU program or Article 80 review
- Multifamily Properties
- Additional accessory units may be permitted in 2F and 3F zones per zoning and state law provisions
Statewide Preemption
ADU Preemption
We could not confirm a statewide ADU preemption statute for Massachusetts.
View Massachusetts preemption detailsFrequently Asked Questions
Can I build an ADU in Boston?
Yes. Boston, MA allows accessory dwelling units. On a single-family lot you can build 1 ADU per single-family lot by-right per Massachusetts Affordable Homes Act (2024); additional units may be possible through Boston's ADU program or Article 80 review.
Source: Boston Zoning Code; Massachusetts General Laws Chapter 40A Section 3; Affordable Homes Act (Chapter 150, Acts of 2024). Last verified April 5, 2026. View source