ADU rules in Massachusetts
Representative cities: Boston, Worcester.
ADU permit rule
Permit requiredGenerally allowedcity authority
Fee basis
Valuation-based; set by the issuing city (Boston uses a long-form permit).
Typical timeline
Boston: up to ~5 weeks for long-form permit review; longer with zoning relief.
Setback
Per local zoning; Boston ADUs may need Zoning Board of Appeal relief if out of dimensional compliance.
Area threshold
State law (MGL c. 40A §3A) allows ADUs by right on lots with a single-family home.
Exempt if
None — an ADU always requires a building permit.
Go / no-go
allowed
Massachusetts requires ADUs by right under MGL c. 40A §3A; Boston requires a long-form building permit and zoning review.
ADU official sources: City of Boston — Permitting Your ADU · Mass.gov — Massachusetts State Building Code (780 CMR)
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Frequently asked questions
Do I need a permit to build an ADU in Massachusetts?
Per Massachusetts rules, an ADU is "yes" for a permit and the authority is "city". Massachusetts requires ADUs by right under MGL c. 40A §3A; Boston requires a long-form building permit and zoning review. Always confirm with the issuing building department.
How big can an ADU be in Massachusetts?
Area threshold: State law (MGL c. 40A §3A) allows ADUs by right on lots with a single-family home.
What are the ADU setbacks in Massachusetts?
Setback: Per local zoning; Boston ADUs may need Zoning Board of Appeal relief if out of dimensional compliance.
How much does an ADU permit cost in Massachusetts?
Fee basis: Valuation-based; set by the issuing city (Boston uses a long-form permit).
State-wide sources: Mass.gov — Massachusetts State Building Code (780 CMR) · City of Boston — Inspectional Services Department · City of Worcester — Building & Zoning
Plan the ADU numbers for Massachusetts
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Rental ROI and payback
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Financing comparison
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