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Brockton, MA Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified September 2026

ADU rules in Brockton, MA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

Adopted: January 30, 2025 (Ord. 01-30-2025(1))
Max size: Lesser of 900 sq ft or half principal home's area
Review required: Site plan review plus separate certificate of occupancy
Detached setback: 10 feet minimum from any other structure
Extra parking: 1 space if over 0.5 miles from transit
Applies where: Only zones allowing single-family dwellings
Rental limit: Not permitted as a short-term rental

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Massachusetts under recent ADU streamlining laws. The Brockton, MA rules below cover local size, height, and setback requirements.

Summary

Brockton allows accessory dwelling units under Zoning Ordinance § 27-35.2, adopted January 30, 2025 to implement the state's 2024 ADU law. Every ADU, attached or detached, is capped at 900 square feet or half the principal home's floor area, needs site plan review and its own certificate of occupancy before anyone can live in it.

(a)[Authority.] Pursuant to Section 8 of Chapter 150 of the Acts of 2024, Accessory Dwelling Units and 760 CMR 71:00 Protected Use Accessory Dwelling Units, this section shall apply to Protected Accessory Dwelling Units in the City of Brockton. ... (e)Standards—All ADUs. All ADUs shall:(1)Require site plan review;(2)Maintain a separate entrance...sufficient to meet the requirement of the state building code for safe egress;...(4)Not be larger in gross floor area than one-half (½) of the gross floor area of the principal dwelling or nine hundred (900) square feet, whichever is smaller;(5)Require a separate certificate of occupancy; and(6)Shall require one (1) additional parking space if located outside one-half (0.5) miles from a commuter rail station, subway station or bus station.

Source: Brockton Code of OrdinancesView official code

Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Code of Ordinances: Supplement 28).

Full Breakdown

Section 27-35.2 implements Section 8 of Chapter 150 of the Acts of 2024 and 760 CMR 71.00, the state's Protected Use ADU law, and applies only in zoning districts where a single-family dwelling is a permitted or allowable use. It defines an attached ADU as one built within the existing single-family home and a detached ADU as a standalone structure, and both are capped at whichever is smaller: half the gross floor area of the principal dwelling, or 900 square feet.

Every ADU requires site plan review, a separate certificate of occupancy, and a separate entrance sufficient to meet the state building code's safe-egress requirements, with exterior stairs kept out of view from the public way. An ADU sited more than half a mile from a commuter rail station, subway station or bus station needs one additional off-street parking space beyond what the principal dwelling already requires.

Attached ADUs must match the principal structure's required front, rear and side setbacks, keep a 10-foot separation from any other accessory structure, and follow the same lot-coverage, green-space and height limits set for the zone in the § 27-9 dimensional table. Detached ADUs can't sit within 10 feet of the principal or another accessory structure, can't be placed closer to the street than the principal dwelling, and on corner lots can't sit between the side street and the front setback line; they must go on the rear or side of the principal structure. Detached ADUs get water and sewer service per Brockton DPW requirements and electric service per the utility provider's requirements. The ordinance also defines an ADU as a unit 'that is not used as a short-term rental.'

Violations & Fines

Occupying an ADU before it receives its own certificate of occupancy, or building one that doesn't meet the § 27-35.2 size, setback or parking standards, is enforced by the inspector of buildings under § 27-51, who won't sign off on a nonconforming structure. Violations carry the city's general penalty of up to $300 per offense under § 1-9.

Frequently Asked Questions

How big can an ADU be in Brockton?

Section 27-35.2(e)(4) caps every accessory dwelling unit, attached or detached, at whichever is smaller: half the gross floor area of the principal dwelling or 900 square feet. The ordinance also requires site plan review and a separate certificate of occupancy before anyone can move in, on top of the standard building permit.

Do I need extra parking for an ADU?

Only if the property sits more than half a mile from a commuter rail station, subway stop or bus station. Section 27-35.2(e)(6) then requires one additional off-street parking space beyond what the principal dwelling already needs, on top of any parking required elsewhere in the zoning ordinance for the district.

Can I rent out my ADU short-term?

The ordinance defines an accessory dwelling unit as a secondary living space 'that is not used as a short-term rental,' so § 27-35.2(b) rules out platforms like Airbnb for the unit itself. Ordinary long-term rental to a tenant is not restricted by this section.

What setback applies to a detached ADU?

A detached ADU can't sit within 10 feet of the principal structure or any other accessory structure, can't stand closer to the street than the principal dwelling, and on a corner lot can't be placed between the side street and the front yard setback line under § 27-35.2(g). It must go on the rear or side of the principal structure.

Sources & Official References

[1]Brockton, MA Code of Ordinances - Appendix C, Zoning (Municode)

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All Brockton rules

How Brockton compares: Easiest Cities to Build an ADU·Compare Brockton to another location·View the Massachusetts accessory structures overview

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