Understand the ADU law in Massachusetts

Mike Guimares February 6, 2025

As of February 2, 2025, Massachusetts homeowners can now build accessory dwelling units (ADUs) by-right in all single-family residential zones—eliminating the need for special zoning approvals. This landmark policy, supported by the Massachusetts Association of Realtors® (MAR), simplifies the development process and expands housing opportunities statewide.

MAR played a key role in shaping these regulations, ensuring they are clear, reasonable, and minimally restrictive. The new rules, issued by the Executive Office of Housing and Livable Communities (EOHLC), override any previous local ADU ordinances and set fair limits on municipal control.

From basement apartments to backyard cottages, ADUs can take various forms, provided they meet size and entry requirements. Importantly, municipalities cannot impose owner-occupancy mandates, excessive parking rules, or restrictive zoning measures that hinder ADU development.

With an estimated 8,000 to 10,000 ADUs expected over the next five years, this policy marks a significant step toward increasing housing accessibility across Massachusetts. Below is an outline provided by MAR (Massachusetts Association of Realtors) to try and bring clarity to resident's rights under this new law.

The final regulations that were released by EOHLC seek to help cities and towns adjust their local ordinances and provide guidance to property owners seeking to build an ADU. Additionally, the regulations supersede any existing ADU ordinances that were adopted by municipalities prior to the law going into effect.

At a high level, the maximum size of an ADU can be no more than half of the gross floor area of the principal dwelling, or 900 square feet, whichever is smaller.ADUs can be internal within an existing primary residence, like converting a basement into an apartment, attached to a primary residence as a new construction addition, or completely detached, like a cottage or converted detached garage in a backyard. Regardless, it must maintain a separate entrance, either directly from the outside or through an entry hall or corridor shared with the principal dwelling.

Municipalities are not permitted to set unreasonable restrictions on the development ADUs, including:

Require owner-occupancy for either the ADU or the principal dwelling.
Impose minimum parking requirements exceeding:
One space if the lot is outside 0.5 miles of a transit station.
No additional spaces if within 0.5 miles of a transit station.
Limit the number of ADUs allowed in a zoning district.
Require ADUs to be only attached or only detached from the main dwelling.

Reasonable municipal regulations must serve a legitimate municipal interest and be proportional to that interest. They cannot impose stricter design, size, or zoning standards than those for single family homes, mandate separate utility connections, or enforce more restrictive environmental or safety rules than state standards. Municipalities can, however, set stricter design rules in historic districts and regulate or prevent ADU’s from being used as short-term rentals.

A more detailed list of specific unreasonable regulations is as follows:

Design Standards

- - They cannot be stricter than those applied to single-family homes.
  - Regulations that increase costs excessively or make ADU construction impractical are not allowed.

Dimensional Standards

- - Setback, lot coverage, open space, bulk, height, and story limits must be equal to or more permissive than those for single-family homes.
  - No minimum lot size can be required for ADU construction.

Utilities, Safety, and Emergency Access

- - No requirement for a separate water, sewer, or electric connection unless mandated by state law or a utility provider.
  - Local emergency access rules cannot be stricter than state fire code.

Environmental Regulations

- - Rules regarding environmental protection (e.g., wastewater treatment, stormwater management) cannot be stricter than those applied to single-family homes in the same zoning district.
  - Must comply with state environmental regulations (Title 5 septic standards, etc.).

Site Plan Review

- - The process must be clear, objective, and fair.
  - Municipalities cannot impose subjective conditions or discretionary reviews that hinder ADU approval.

Impact Analysis, Studies, and Fees

- - Municipalities cannot require extra studies, reports, or fees for ADUs if these are not also required for single-family homes.

Modular Dwelling Units

- - Prefabricated or modular homes used as ADUs must be allowed if they comply with the Massachusetts Building Code.
  - Cities cannot impose additional restrictions just because an ADU is modular.

Historic Districts

- - Municipalities may impose stricter design and dimensional standards on ADUs in historic districts.
  - However, these standards must still be reasonable and cannot outright prevent ADU construction.

Pre-existing Nonconforming Structures

- - If a principal dwelling, existing structure, or lot is already nonconforming with zoning rules (e.g., due to setback violations), a Protected Use ADU cannot be prohibited solely for this reason.
  - The ADU must be allowed if it complies with the Building Code and state law.

Need help with weighing your options and getting connected with the right builder that can make your ADU needs a reality? Contact us today!