Massachusetts General Law (M.G.L. c. 40A, § 9B) now mandates that municipalities permit the creation of one Accessory Dwelling Units (ADUs) on lots currently occupied by a single-family dwelling. Key provisions of this statewide requirement include:
- By-Right Approval: Qualifying ADUs must be permitted "by right," meaning they are not subject to discretionary review processes such as special permits or variances. Approval typically requires only the issuance of a standard building permit, contingent upon compliance with applicable codes.
- Permitted Locations: ADUs may be established either within the existing footprint of the primary single-family residence or within a detached structure (new or existing) located on the same lot.
- Maximum Size: The gross floor area of the ADU may not exceed the lesser of 900 square feet or 50% of the principal dwelling's gross floor area.
- Use Restriction: The ADU must be utilized solely for residential purposes.
- Code Compliance: All ADUs must adhere to relevant state and local regulations, including building, health, safety, and sanitation codes (e.g., Title 5 for septic systems).
- Limitations on Local Restrictions: Municipalities are prohibited from imposing unreasonable regulations concerning the size, siting, or design of by-right ADUs that would effectively preclude their development. Furthermore, owner-occupancy requirements cannot be mandated for ADUs permitted under this specific state provision.
Notwithstanding the statewide by-right provisions, municipalities retain substantial authority to regulate ADUs through local zoning bylaws. For local residents, this means:
- Supremacy of State Law for By-Right Units: Local bylaws cannot impose requirements more restrictive than M.G.L. c. 40A, § 9B for ADUs meeting the state criteria.
- Option for Greater Permissiveness: Local zoning bylaws may allow for ADU development under conditions more permissive than the state minimums (e.g., allowing larger units, permitting ADUs on multi-family lots, or allowing more than one ADU per lot).
- Regulation of Non-Conforming ADUs: Proposed ADUs that do not meet the specific criteria for by-right approval under state law are subject entirely to local zoning regulations. These regulations typically address parameters such as dimensional requirements (setbacks, height), parking standards, design guidelines, and potentially require discretionary review (e.g., special permit).
The Commonwealth of Massachusetts has established a mandatory baseline facilitating the development of one ADU per single-family lot under specified conditions. However, the comprehensive regulatory landscape for ADUs is defined by the interplay between state law and local zoning bylaws.
Property owners contemplating the development of an ADU must consult their local municipality’s current Zoning Bylaws and confer with the relevant municipal departments to ascertain the specific requirements, limitations, and procedures applicable to their property and proposed project.
This content is for general informational purposes only, not legal advice. For assistance with your specific legal needs, please consult with one of our attorneys.