Massachusetts ADU permit requirements
Last reviewed: June 20, 2026
Quick answer
Permitting an accessory dwelling unit (ADU) in Massachusetts means pulling a building permit from the local authority that issues it. Massachusetts has a statewide ADU law that sets baseline standards local agencies must follow. ArchPermit tracks 14 source-cited requirements (12 required, the rest conditional) for this jurisdiction. These requirements were last reviewed June 20, 2026; confirm the exact forms and fees with the local permitting authority.
Massachusetts has a statewide ADU law. This baseline reflects that law plus the universal new-dwelling deliverables. Your local city or county still administers the permit and may add its own forms and fees — confirm the exact checklist with them.
Massachusetts' Affordable Homes Act (Ch. 150 of 2024, effective Feb 2, 2025) allows ADUs up to 900 sq ft (or half the principal dwelling's GFA) by right in single-family districts, bars owner-occupancy requirements, and bars required parking within ½ mile of transit. The local AHJ administers permits and may require site-plan review and dimensional standards.
- ADU ≤ 900 sf or half the principal (by-right)required
To qualify as by-right in single-family districts, an ADU must be ≤900 sq ft or half the principal dwelling's gross floor area, whichever is smaller.
source · reviewed 2026-06-20 - Adopted building code compliancerequired
The ADU must comply with Massachusetts's adopted residential building code: Massachusetts State Building Code (780 CMR), 10th ed.. Confirm the in-force edition with the local AHJ.
source · reviewed 2026-06-20 - Building cross-sectionsrequired
Cross-sections through the structure showing ceiling heights, wall/roof assemblies, insulation, and fire/sound-rated assemblies where required.
source · reviewed 2026-06-20 - Egress & life-safetyrequired
Emergency escape and rescue openings (egress windows), smoke and carbon-monoxide alarms, and required exits per the adopted residential code.
source · reviewed 2026-06-20 - Energy code compliancerequired
Energy-code compliance documentation for the conditioned space. Massachusetts: Base: 2021 IECC (780 CMR 10th ed.); optional Stretch/Specialized codes.
source · reviewed 2026-06-20 - Exterior elevationsrequired
Elevations of each affected building face showing height, exterior materials, and openings; existing vs. proposed where exterior changes occur.
source · reviewed 2026-06-20 - Floor plansrequired
Dimensioned floor plans of the ADU (and affected areas of the primary dwelling) showing rooms, uses, door/window locations, egress, and fixture/equipment locations.
source · reviewed 2026-06-20 - Foundation planrequired
Foundation plan with footing/stem-wall sizes and details (or a cited site-specific design), per the adopted residential code.
source · reviewed 2026-06-20 - Framing / structural plansrequired
Floor, wall, and roof framing showing member sizes, spans, spacing, connections, and lateral bracing; structural calculations where the framing is non-conventional.
source · reviewed 2026-06-20 - Local zoning / setback compliancerequiredverify locally
Compliance with the local jurisdiction's zoning: permitted use, setbacks, height, lot coverage, and any ADU-specific standards. These are set locally — confirm with the permitting agency.
source · reviewed 2026-06-20 - Mechanical / plumbing / electricalrequired
HVAC, plumbing fixtures and lines, and electrical (panels, circuits, outlets, lighting, smoke/CO alarms) for the new unit.
source · reviewed 2026-06-20 - Site / plot plan (to scale)required
Scaled site plan showing property lines, setbacks, existing structures, the proposed ADU footprint with dimensions to property lines, driveways/parking, and utility locations.
source · reviewed 2026-06-20 - No parking required within ½ mile of transitconditionalverify locally
No parking may be required for an ADU within ½ mile of a transit station; elsewhere at most one space.
source · reviewed 2026-06-20 - Owner-occupancy may not be requiredconditional
Municipalities may not require owner occupancy of the ADU or principal dwelling.
source · reviewed 2026-06-20