Side by side · Massachusetts
Compare ADU rules across Massachusetts
Flexible on where you build? The same backyard cottage can be routine in one city and impossible next door. This table puts 2 Massachusetts cities side by side so you can shortlist the friendly ones before reading a single ordinance, or narrow it to just the cities you are deciding between. Every value is summarized from the city's own code; open a city's page for the full notes, citations, and fine print.
The short answers
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| City | Detached ADU | Attached / internal | Max size | Owner occupancy | Extra parking | Approval |
|---|---|---|---|---|---|---|
| Boston | 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))Full rules for Boston Ordinance read July 2026 | Allowed | 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 | Required | 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 | 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 8-2.2Full rules for Boston Ordinance read July 2026 |
| Cambridge | Allowed | Allowed | No city-specific ADU cap in the current ordinance; detached (state protected-use) ADUs are capped by state law at 900 sq ft or half the principal dwelling's gross floor area, whichever is smaller | Not required | None; minimum accessory parking for all uses is zero citywide | Permitted use |
Summarized from each city's ordinance text, most recently read July 2026. Tap a value marked with an info icon to see exactly what it depends on, straight from the ordinance. Nearly every value has zone, lot-size, or process caveats beyond that; the per-city pages carry the full notes and code citations. Treat this as orientation, not a permit determination.
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Reading the table
- Detached ADU is the make-or-break column for backyard cottages: minimum lot sizes (often 8,000 to 14,500 sq ft) hide behind many of the "Allowed" values, so check your lot before celebrating.
- Attached / internal (basement apartments, additions) is legal almost everywhere and is usually the path of least resistance.
- Approval: "Permitted use" means staff-level sign-off; "Conditional use" means a public hearing, which adds time, cost, and uncertainty.
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