The Future of ADUs in Massachusetts
1. Landmark Legislation: The Affordable Homes Act of 2024
On August 6, 2024, Governor Maura Healey signed the Affordable Homes Act (Chapter 150 of the Acts of 2024). A key provision—Section 8—amends the Massachusetts Zoning Act (G.L. c. 40A), enabling homeowners to build ADUs “by right” in single-family zoning districts.
Key limits:
- Max size: 900 sq. ft. or 50% of the main home, whichever is smaller
- No special permits required
- No owner-occupancy requirements
- No excessive parking requirements, especially near transit
2. Rollout of Final Regulations and Timeline
- Effective Date: February 2, 2025
- Final ADU regulations: Published in January 2025 under 760 CMR 71.00
- State projection: 8,000–10,000 ADUs may be built in the next five years
3. Types of ADUs Allowed
Types of ADUs Allowed in Massachusetts (2025)
- Interior: basement, attic, or within existing walls
- Attached: over a garage or added onto the main home
- Detached: backyard cottage or garage conversion
4. Key Provisions and Their Impact
- Local Restrictions Allowed: site plan reviews, septic (Title V), setbacks, dimensional rules
- Local Restrictions Not Allowed: owner-occupancy mandates, excessive parking, unnecessary utility separations
- Septic Systems: Homes on private septic may need Title V upgrades if bedrooms are added
Exceptions: Boston maintains its own ADU rules and is not governed by the new law.
5. Projected Growth of ADUs
Projected ADU Growth in Massachusetts (2025–2029)
- 2025: ~1,000 units
- 2026: ~2,500 units
- 2027: ~4,500 units
- 2028: ~7,000 units
- 2029: ~10,000 units
6. Practical Takeaways for Homeowners
Consideration
Implication
Building by Right
One ADU per single-family lot, no special permits
Parking
One space may be required, but not within ½ mile of transit
Owner-Occupancy
Not required – ADUs can be rented
Septic
Title V upgrades may be needed for added bedrooms
Local Bylaws
Still apply – towns may regulate design, size, and review
7. Looking Ahead
- More ADUs expected: Thousands statewide in the next five years
- Municipal variation: Towns updating bylaws now (e.g., Grafton, North Andover)
- Policy support: State and advocacy groups offering model bylaws and homeowner resources
- Next wave: Possible easing of size limits, septic flexibility, and more technical assistance
Thinking about adding an ADU to your property in Massachusetts? W. H. Lyon Builders can help you evaluate feasibility, navigate permitting, and build to last. Start the conversation at wh-build.com.
Bibliography
Mass.gov, “Accessory Dwelling Units” (2025).
Link: https://www.mass.gov/info-details/accessory-dwelling-units
Mass.gov, “Accessory Dwelling Units Officially Allowed Statewide” (2025).
Link: https://www.mass.gov/news/accessory-dwelling-units-officially-allowed-statewide
MBCIA, “New ADU Regulation 760 CMR 71.00 Takes Effect” (2025).
Abundant Housing MA, “Statewide ADU Implementation” (2025).
Link: https://www.abundanthousingma.org/statewide-adu-implementation
MBM Law, “Changes to the Zoning Law Governing ADUs” (2024).
Link: https://www.mbmllc.com/zoning-law-change-accessory-dwelling-units-massachusetts.html
Vanderveen Law, “What You Need to Know About Massachusetts’ New ADU Law” (2025).
Grafton, MA Planning Department, “Accessory Dwelling Units—Section 8” (2025).
Link: https://www.grafton-ma.gov/1130/Accessory-Dwelling-Units-ADU---Section-8
North Andover, MA, “Accessory Dwelling Units” Community Forum (2025).
Link: https://www.northandoverma.gov/372/Accessory-Dwelling-Units
Four Generations One Roof, “Massachusetts ADU Regulations 2025” (2025).
Blue Green Building Concepts, “Massachusetts ADU Guide 2025” (2025).
McGregor Law, “Massachusetts’ New ADU Law” (2025).
