City · Massachusetts

ADU rules in Boston

4 sources

These are the primary sources for accessory dwelling unit regulations in Boston: the city's own zoning code, planning department pages, and permit guides. Start here before talking to anyone: the rules below are what your permit reviewer will apply.

Rules at a glance

Detached (backyard) ADU

Conditional

By right only in the Greater Mattapan Neighborhood District residential subdistricts R1, R2, 2F, and 3F (forbidden in MFR); conditional use in Squares + Streets S0 and S1 districts; forbidden as a listed use everywhere else (East Boston tables mark it F in all subdistricts), so elsewhere a detached ADU needs ZBA relief. Detached ADU standards: floor plate no more than 900 sq ft or the primary structure's floor plate (whichever is less), height max 1.5 stories and no taller than the primary structure, one per lot, 5 ft from other structures, may be built in the rear yard in Mattapan.

Boston Zoning Code Art. 60 Table A (accessory uses) and Art. 8 Table A; standards in Section 2-1 (definition, Accessory Dwelling Unit (Detached))

Attached or internal ADU

Allowed

Internal-conversion Additional Dwelling Units are an allowed use in residential subdistricts across Boston's neighborhood district articles even where otherwise conditional or forbidden, for owner-occupied structures of up to 3 units, adding no more than 1 unit entirely within the existing building envelope (identical language in Sections 50-28, 51-8, 53-5, 54-9, 55-8, 56-7, 59-7, 61-7, 62-6, 65-8, 66-8, 67-8, 68-7, 69-8). Non-detached ADUs with bump-outs or additions are a distinct use allowed by right only in Mattapan R1/R2/2F/3F and conditionally in Squares + Streets S0/S1.

Boston Zoning Code Section 56-7.3 (representative; same provision in other district articles); Section 2-1 (definitions, Additional Dwelling Unit and Accessory Dwelling Unit (non-Detached))

Maximum size

Internal Additional Dwelling Unit: no added floor area (must fit within the existing envelope). Non-detached ADU: addition capped at 75% of the principal unit's gross square footage or 1,250 sq ft, whichever is smaller. Detached ADU: floor plate max 900 sq ft or the primary structure's floor plate, whichever is less; max 1.5 stories

The non-detached and detached ADU caps only matter in districts where those uses are listed (Mattapan, Squares + Streets S0/S1).

Boston Zoning Code Section 2-1, definitions of Accessory Dwelling Unit (non-Detached) and Accessory Dwelling Unit (Detached)

Owner occupancy

Required

Every ADU pathway requires the owner on site: non-detached ADUs require the Owner to be a principal resident, detached ADUs require the Owner to be a resident of the lot, and the internal Additional Dwelling Unit exemption applies only while the structure is owner-occupied and registered with the city rental registry.

Boston Zoning Code Section 2-1 (ADU definitions); Section 56-7.3

Extra parking

No parking required for ADUs. Internal Additional Dwelling Units are exempt from all code requirements (including parking) when within the existing envelope; detached and non-detached ADUs are exempt from Section 23-9.a (location of accessory parking), and Mattapan's parking table sets 0 spaces per Accessory Dwelling Unit

Mattapan Table H: Accessory Dwelling Unit, 0 spaces per dwelling unit.

Boston Zoning Code Section 2-1 (ADU definitions); Section 56-7.3; Art. 60 Table H

Approval process

Varies by type

As-of-right for internal conversions in residential subdistricts citywide (owner-occupied, up to 3 existing units, no envelope expansion); as-of-right for detached and non-detached ADUs only in Greater Mattapan R1/R2/2F/3F; conditional use permit (Board of Appeal, Article 6) in Squares + Streets S0/S1; ZBA variance needed for detached ADUs in the rest of the city. Building permits go through the Inspectional Services Department.

Boston Zoning Code Section 56-7.3 (and parallel district sections); Art. 60 Table A; Art. 8 Table A and Section 8-2.2

Rental restrictions

ADUs are defined as non-transient dwelling units, which excludes short-term rental use; the residential structure must be registered under the City of Boston Rental Registry Ordinance (Ch. 9-1.3) at the time of conversion

No minimum lease term is stated in the zoning code itself; the non-transient requirement and rental registry hook are in the ADU definitions.

Boston Zoning Code Section 2-1, definitions of Accessory Dwelling Unit (non-Detached), Accessory Dwelling Unit (Detached), and Additional Dwelling Unit

The fine print: 6 caveats that can change the picture

The statewide ADU by-right law does NOT apply in Boston: the Affordable Homes Act (St. 2024, c. 150, s. 8) amended MGL c. 40A s. 3, but Boston is not subject to c. 40A (its zoning runs under its own enabling act, Ch. 665 of the Acts of 1956, which is printed at the front of the Boston Zoning Code), and 760 CMR 71.02 defines Municipality as 'any city or town subject to the provisions of M.G.L. c. 40A'. Boston ADUs are governed solely by the Boston Zoning Code.
There is no single ADU article in the Boston Zoning Code (Article 89 is Urban Agriculture, not ADUs). ADU rules are split between the Section 2-1 definitions, per-district use tables, and identical Additional Dwelling Unit provisions in each neighborhood district article.
Allowances are heavily district-dependent: detached and non-detached ADUs are by right only in Greater Mattapan residential subdistricts R1/R2/2F/3F (forbidden in MFR), conditional in Squares + Streets S0/S1, and forbidden as listed uses elsewhere (including all East Boston subdistricts).
The citywide internal-conversion path (Additional Dwelling Unit) only covers owner-occupied structures of up to 3 units, adds at most 1 unit, and must stay entirely within the existing building envelope; any bump-out or addition takes the project out of the exemption.
Some districts forbid basement dwelling units outright (West Roxbury Sec. 56-7.2, Dorchester Sec. 65-8.2), while Mattapan expressly allows ADUs in basements (Art. 60 Table A footnote 4).
Boston is actively rezoning: the Planning Department's Neighborhood Housing initiative aims to make ADUs as-of-right citywide, and the code is amended frequently (codified through Text Amd. No. 494, effective 1-12-2026), so district allowances may change.

Summarized from Boston's ordinance text, read July 2026; every row links the code section it came from. Rules often vary by zone and lot, so treat this as orientation, not a permit determination, and confirm with the planning department before you commit to a design.

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ADU regulations

Massachusetts statewide law also applies

Massachusetts sets baseline ADU rights at the state level. Where local rules are stricter than state law allows, state law usually wins, so both are worth reading.

See Massachusetts statewide rules

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