How to Add an Accessory Dwelling Unit (ADU) in Massachusetts: Legal Requirements & Construction Tips

Understanding What Qualifies as an ADU and How to Build One the Right Way in MA

ADU's are all the rage.

If you're a Massachusetts homeowner looking to expand your property's potential, adding an Accessory Dwelling Unit (ADU) is a great option. Also known as in-law suites, granny flats, or secondary units, ADUs are small, independent living spaces located on the same lot as a primary residence. They provide an excellent opportunity for multi-generational living, rental income, or dedicated office space. However, Massachusetts law places specific requirements on what constitutes a legal ADU, and it’s essential to understand both state and local regulations before you build.

What Legally Defines an Accessory Dwelling Unit (ADU) in Massachusetts?

In Massachusetts, ADUs are primarily regulated at the municipal level, although they must comply with state building codes. Generally, an ADU must:

• Be secondary to the principal dwelling on the lot

• Include its own kitchen, bathroom, living area, and sleeping space

• Have a separate entrance

• Be permanently affixed to a foundation (in most cases)

• Be located on the same lot as the primary home

• Be owner-occupied in either the main or accessory unit in many towns

• Meet the town’s zoning requirements (use, size, setback, and height restrictions)

Some municipalities place size limits on ADUs (e.g., maximum 900 sq. ft. or 50% of the main house), and some may restrict them to attached units (within or connected to the existing home), while others allow detached units (like converted garages or standalone backyard cottages).

Key Legal and Zoning Considerations

To legally construct an ADU in Massachusetts, you'll need to:

1. Review Local Zoning Bylaws: Not all cities and towns in Massachusetts allow ADUs by right. In some areas, you may need a special permit or zoning board approval.

2. Meet Building Code Requirements: ADUs must conform to 780 CMR (Massachusetts State Building Code), including structural, electrical, plumbing, fire safety, and energy efficiency standards.

3. Address Septic or Sewer Requirements: If your home uses a septic system, the system must be able to accommodate the additional flow from an ADU, and you may need approval from the local Board of Health.

4. Comply with Historic District Rules (if applicable): Homes in designated historic zones may be subject to architectural review.

Steps to Build an ADU

1. Feasibility Assessment: Consult with a licensed contractor and/or architect to determine if your property and existing structure support an ADU.

2. Design and Planning: Create architectural drawings that meet local zoning and state code. Include plans for access, utilities, and required amenities.

3. Permit Application: Submit all required documents to your local Building Department, which may include floor plans, plot plans, and engineering reports.

4. Construction: Hire a reputable, fully licensed contractor like Thomson’s Home Improvement to ensure your ADU is built to code.

5. Inspection and Occupancy: Once construction is complete, the ADU must pass final inspections and receive a Certificate of Occupancy before it can be used.

Why Work with Thomson’s Home Improvement?

Thomson’s Home Improvement is a fully licensed and insured contractor serving Randolph, MA and the surrounding South Shore communities. We have the experience and local knowledge to guide homeowners through the entire ADU process, including:

• Navigating local zoning and permitting

• Providing expert design and layout services

• Managing all aspects of construction

• Ensuring code compliance and final approvals

Final Thoughts

Adding an ADU can increase your home’s value, provide rental income, and create flexible living space for your family. But because Massachusetts law and local zoning rules can be complex, it’s important to work with professionals who understand the process.

Contact Thomson’s Home Improvement today for a free consultation on building your ADU.

📞 857-719-2636

🌐 www.thomsonshome.com

Serving Randolph, Milton, Quincy, Braintree, Canton, Stoughton, Brockton, Avon, Holbrook, Weymouth, Abington, Rockland, Hingham, Norwell, Hanover, Pembroke, Whitman, Hanson, East Bridgewater, West Bridgewater, Bridgewater, Easton, Sharon, Dedham, Norwood, Westwood, Walpole, Foxborough, Mansfield, Medfield, Dover, Needham, Newton, Brookline, Boston, Mattapan, Hyde Park, Roslindale, Jamaica Plain, Dorchester, South Boston, Hull, Cohasset, Scituate, Marshfield, Waltham, Watertown, Belmont, Lexington, Arlington, Cambridge, Somerville, Everett, Chelsea, Revere, Malden, Medford, Melrose, Wakefield, Stoneham, Reading, Woburn, Burlington, Winchester, and surrounding towns.