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
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).
