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 15 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.
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| City | Detached ADU | Attached / internal | Max size | Owner occupancy | Extra parking | Approval |
|---|---|---|---|---|---|---|
| Amherst | Allowed | Allowed | 900 sq ft or half the principal dwelling's gross floor area, whichever is smaller, by right; up to 1,200 sq ft as a 'Local ADU' with administrative approval | It depends entirely on which track you use. A Protected Use ADU carries no owner-occupancy condition: state law forbids one ('The use of land or structures for an accessory dwelling unit under this paragraph shall not require owner occupancy of either the accessory dwelling unit or the principal dwelling', MGL c.40A s.3; 760 CMR 71.03(2)(a)), and Section 5.0114 imposes none. A Local ADU, the oversize track above the protected cap and up to 1,200 sq ft, does require it: Section 5.0115(a) says either the principal dwelling or the Local ADU 'shall be occupied by the owner(s) as their Principal Residence', made a condition of the Building Commissioner's approval and locked in by a deed restriction filed with the Registry of Deeds. Principal residence is defined at 12.41 and is determined on a preponderance of evidence (homestead declaration, tax filings, voter registration, license, and so on). Before the 2025 rewrite, owner occupancy applied to every accessory unit in town. Ordinance read August 2026 | None for a protected ADU within 1/2 mile of a commuter rail station or bus stop; at most 1 space otherwise; 2 spaces for a Local ADU unless the Building Commissioner approves a lower ratio | Permitted use |
| Barnstable | Allowed | Allowed | 900 sq ft or 1/2 the principal dwelling's gross floor area, whichever is smaller; above 900 sq ft only by ZBA special permit and only inside an existing structure | Not required | Up to 1 additional off-street space; none required within 0.5 miles of a commuter rail station, subway station, ferry terminal or bus station | Permitted use |
| 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 |
| Brockton | Allowed | Allowed | 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is smaller | Not required | 1 space, none within 0.5 miles of a commuter rail, subway or bus station | The ADU use itself is by right (no special permit is legally possible), but Brockton is not a counter-transaction city: Sec. 27-35.2(e)(1) requires site plan review for every ADU, attached or detached, and under Article XV the permit granting authority is the planning board. Sec. 27-87 requires notice by publication as for a special permit and a vote of the majority plus one of the planning board members attending, and Sec. 27-86 lets the board attach "the conditions and special conditions deemed necessary and appropriate" while stating the permit "shall not be considered a special permit subject to the procedural requirements of M.G.L. chapter 40A." The city's March 2026 site plan review application packet confirms the practice, listing "Accessory Dwelling Unit (ADU)" as a threshold and describing a technical review session with city departments followed by a Planning Board public hearing on the first Tuesday of the month. Expect a monthly filing deadline, engineered and stamped plans, and a hearing; do not expect a discretionary denial, because the use is protected. A separate certificate of occupancy is required under Sec. 27-35.2(e)(5), on top of building permits under 780 CMR and 527 CMR. Ordinance read August 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 |
| Easthampton | Allowed | Allowed | 900 sq ft for new construction (detached or attached); no square-foot cap when converting an existing accessory structure | Easthampton's ordinance still says yes, but state law makes it unenforceable in the ordinary case. Section 8.53(b), unamended 2021 text, reads: "The owner(s) of the detached single- or two-family dwelling shall occupy one of the units as their primary residence at the time of occupancy, except for bona fide temporary absences. A property containing an approved ADU may be sold to a new owner even if it is non-owner occupied." Since February 2, 2025, M.G.L. c. 40A, s. 3 has provided that use of land or structures for a protected ADU "shall not require owner occupancy of either the accessory dwelling unit or the principal dwelling," and 760 CMR 71.03(2)(a) lists owner-occupancy requirements as a Prohibited Regulation. So for the normal case, one ADU no larger than the smaller of 900 sq ft or half the principal dwelling's gross floor area, on a lot with a single-family house in R-5 through R-80, no owner occupancy may be required. The local clause still reads on configurations Easthampton allows but the state does not protect: an ADU accessory to a two-family dwelling (760 CMR 71.02 defines a Principal Dwelling as a structure containing not more than one dwelling unit), and a unit larger than the state's protected size, which section 8.55(a) permits without limit when an existing accessory structure is converted. Zoning Ordinance (Appendix G) § 8.53(b) vs. M.G.L. c. 40A, § 3, para. 11 and 760 CMR 71.02, 71.03(2)(a)Full rules for Easthampton Ordinance read August 2026 | 1 additional off-street space, waivable in writing; none required within 0.5 mile of a transit stop under state law | Permitted use |
| Lowell | Allowed | Allowed | 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is smaller | Not required | None within 0.5 miles of a transit station (Lowell counts bus stops, which covers most of the city); at most 1 space anywhere else | Permitted use |
| New Bedford | Allowed | Allowed | 900 sq ft or half the principal dwelling's gross floor area, whichever is smaller, by right; up to 1,200 sq ft by Zoning Board of Appeals special permit | Not required | 1 off-street space, but none at all within a half mile of a transit station (which includes any SRTA bus stop) | Permitted use |
| Newton | Allowed | Allowed | 1,000 sq ft by right (internal or detached); 1,200 sq ft by right in defined cases; up to 1,200 sq ft internal or 1,500 sq ft detached by City Council special permit; in every case no more than 50% of the combined ADU plus principal-dwelling floor area | Not required | None | Permitted use |
| Northampton | Allowed | Allowed | 1,800 sq ft cap on one of the two dwellings in RR, SR, URA and WSP (detached configuration); no size cap on an attached second unit; a 'half-scale' unit is 800 sq ft or less | Not required | 1 space per unit, so one additional off-street space for the second dwelling | Permitted use |
| Pittsfield | Allowed | Allowed | 900 sq ft, or half the principal dwelling's gross floor area, whichever is smaller | Not required | 1 space, and none if any part of the lot is within 1/2 mile of a transit station | Permitted use |
| Quincy | Allowed | Allowed | 900 sq ft or 50% of the principal structure's gross floor area, whichever is less; detached ADUs must be at least 320 sq ft and no more than one story | Not required | 1 space if the lot is more than 0.5 mile from a transit station; none within 0.5 mile, and existing off-street parking may not be reduced | Permitted use |
| Somerville | Allowed | Allowed | 576 sq ft maximum floor plate, 1.5 stories, 24 ft wide by 32 ft deep, 1 dwelling unit (Backyard Cottage); a state protected ADU is separately capped at 900 sq ft or half the principal dwelling's gross floor area, whichever is smaller | Not required | None. Somerville sets no motor-vehicle parking minimum for residential uses and none at all for accessory uses | Permitted use |
| Springfield | Allowed | Allowed | 900 sq ft or half the principal dwelling's gross floor area, whichever is smaller; new detached units capped at 20 ft tall | Not required | One on-site space, but state law bars any parking requirement within 0.5 miles of a transit station | Permitted use |
| Worcester | Allowed | Allowed | 900 sq ft gross floor area (flat cap, no percentage-of-house test); more by Planning Board special permit | Not required | None; existing required parking may not be displaced | Permitted use |
Summarized from each city's ordinance text, most recently read August 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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