ADU rules in Brockton

Detached backyard ADUs are allowed as of right. Brockton got ahead of the state deadline: Ord. No. 01-30-2025(1), adopted January 30, 2025, three days before the c. 40A ADU amendments took effect, added Sec. 27-35.2 (Accessory dwelling units) and listed "Accessory dwelling units" as a permitted accessory use in the R-1 single-family zones under Sec. 27-25(2)(c), which R-2 and R-3 pick up by reference. Sec. 27-35.2(c) applies the section to any zoning district where single-family residential dwellings are a permitted or an allowable use, matching the state definition. Sec. 27-19 allows one principal dwelling structure, one ADU (attached or detached), and two other accessory structures per lot in R-1A, R-1B and R-1C. Siting rules for the detached case: rear or side of the principal structure only, never closer to the street than the principal structure, at least 10 ft from any principal or accessory structure, and off the side-street side on corner lots.

Attached and internal ADUs are allowed on the same by-right footing, but Brockton's definition is unusually narrow: "An attached ADU is built within the existing single-family dwelling." Read literally that covers internal conversions (a basement, an upper floor, space inside the existing house) and does not clearly cover an ADU built as a new attached addition, which also does not fit the detached definition ("a standalone structure separate from the single-family dwelling"). The city's own 2024 ADU flyer had described attached ADUs as "within the existing structure or an addition," so the gap looks like drafting rather than policy. Attached ADUs take principal-structure setbacks and height from Sec. 27-9 Table 1, must sit no closer to the street than the principal structure, must keep a 10 ft setback from any other accessory structure, and must run all utilities through the principal structure.

Sec. 27-35.2(e)(4) copies the state protected-use floor verbatim and stops there: no ADU may be larger in gross floor area than one-half of the principal dwelling's gross floor area or 900 sq ft, whichever is smaller. Brockton chose not to be more generous, which it was free to do under 760 CMR 71.03(7). For the 900 sq ft calculation the state definition of gross floor area governs (760 CMR 71.02): it counts basements, lofts and intermediate floored tiers of code-compliant ceiling height and excludes crawl spaces, garage parking areas, attics and enclosed porches. That is broader than Brockton's own zoning definition of "floor area," which excludes cellar space, stairways and any space under 7 ft, so do not size the unit off the local definition. Height: attached ADUs get the principal-structure limit in Sec. 27-9 Table 1 (2½ stories or 35 ft in R-1A, R-1B and R-1C); for detached ADUs Sec. 27-35.2(g)(7) points at Table 1 without naming a row, and Table 1 caps accessory buildings at 20 ft against 35 ft for principal buildings.

Sec. 27-35.2 contains no owner-occupancy clause, and none could be imposed. M.G.L. c. 40A, § 3 provides that the use of land or structures for an ADU "shall not require owner occupancy of either the accessory dwelling unit or the principal dwelling," and 760 CMR 71.03(2)(a) lists an owner-occupancy requirement as a Prohibited Regulation that a municipality "shall not" apply to a Protected Use ADU. Neither the house nor the ADU has to be owner-occupied, at application or afterwards, and 760 CMR 71.03(4) bars enforcement of owner-occupancy conditions attached to ADU approvals granted before the regulation took effect. Brockton's own 2024 ADU flyer said the same thing: "Principal building or ADU do NOT have to be owner-occupied."

Sec. 27-35.2(e)(6) requires one additional parking space for an ADU only "if located outside one-half (0.5) miles from a commuter rail station, subway station or bus station," tracking the state cap in M.G.L. c. 40A, § 3. Watch the definition: Brockton defines a bus station as "a place where buses start and end their routes," which is narrower than 760 CMR 71.02, where a Bus Station is "a location serving as a point of embarkation for any bus operated by a Transit Authority," and than EOHLC's FAQ gloss (a regular and specific pick-up and drop-off location, excluding flag stops). The state definition controls, so a BAT or MBTA stop that is not a route terminus can still zero out the requirement. The state rule is also measured generously: 760 CMR 71.03(2)(b)(2) bars any additional space if any portion of the lot falls within a 0.5 mile radius of a transit station. Sec. 27-9 Table 1's generic one-space-per-housing-unit standard cannot be stacked on top of this.

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.

Renting the ADU long term is fine and cannot be restricted: M.G.L. c. 40A, § 3 protects "the use of land or structures for a single accessory dwelling unit, or the rental thereof." Short-term rental is out, and it is built into the definition rather than a separate rule: Brockton defines an ADU as a secondary living space "that is not used as a short-term rental," a restriction the state expressly authorizes (760 CMR 71.03(3)(c) and c. 40A, § 1A). Short-term rental means the M.G.L. c. 64G, § 1 definition, which EOHLC reads as stays of not more than 31 consecutive days, so month-to-month tenancies and tenancies at will are outside it. Separately, any non-owner-occupied unit in Brockton is a "rental unit" that must be registered with the Public Health Division by July 1 each year (Sec. 4-21) and inspected at least once every three years (Sec. 4-23); the fee is $30 to register and $20 per unit annually, waived (registration still required) for owner-occupied dwellings with two or fewer rental units, and a newly built unit is exempt from the article for five years after its certificate of occupancy (Sec. 4-26(b)). Nothing in Sec. 27-35.2 addresses condominium conversion or separate sale.