MA ADU GUIDE

THE MASSACHUSETTS ADU GUIDE

Understand the statewide ADU rules — and what still depends on your town and property.

A practical, source-backed starting point for homeowners evaluating an ADU in Massachusetts. Enter an address for property-specific information, or continue to the statewide rules below.

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60-Second Answer

THE 60-SECOND ANSWER

One protected ADU is allowed by right in qualifying single-family residential districts.

Massachusetts law prevents municipalities from prohibiting or unreasonably restricting one qualifying ADU, or requiring a special permit or other discretionary zoning approval for it. The rule does not erase building, fire, health, environmental, utility, dimensional, or reasonable local requirements.

SIZE

Up to 900 sq. ft. — or half the principal dwelling

The protected-use definition caps gross floor area at the smaller of 900 square feet or 50% of the principal dwelling’s gross floor area. A municipality may impose additional reasonable size restrictions.

APPROVAL

No special permit for the protected use

Site plan review and reasonable local standards may still apply. Confirm the zoning district, permit path, and current municipal rules before designing.

OCCUPANCY

No owner-occupancy or family-relationship rule

A municipality may not require the owner to live in either unit or require an occupant to be related to the owner for a protected-use ADU.

PARKING

Parking requirements are limited

No required ADU parking is allowed within 0.5 miles of specified transit stations. Outside that area, a municipality may not require more than one ADU parking space.

FORM

Interior, attached, or detached

A qualifying ADU may be created within the principal dwelling, as an addition, or as a detached structure on the same lot, subject to applicable standards.

SEPTIC & UTILITIES

Property systems still matter

Title 5 and reasonable local health requirements may require septic review or upgrades. Sewer, water, electrical, and site conditions can materially affect feasibility and cost.

Current-law note: This guide reflects the statute and 760 CMR 71.00 currently in effect. EOHLC opened public comment on proposed 2026 revisions; proposals are not treated here as adopted law.

WHERE TO START

Begin with the property, not a floor plan.

Confirm the principal dwelling type and zoning district, identify whether the ADU would be interior, attached, or detached, estimate the likely size, and determine whether the property has public sewer or private septic. These facts shape the questions worth asking first.

  • Property address and zoning district
  • Principal dwelling gross floor area
  • Likely ADU location and approximate size
  • Existing structures, access, slope, wetlands, and setbacks
  • Sewer or septic, water service, and electrical capacity
  • Intended use, timeline, and early budget range

STATE VS. LOCAL RULES

The state creates a protected baseline; municipalities administer the details.

Local zoning cannot prohibit or unreasonably restrict a protected-use ADU, but municipalities may apply reasonable site plan review, dimensional and design standards, short-term-rental restrictions, and other requirements consistent with state law and regulation. Local boards also administer building permits and, where applicable, septic and health requirements.

Do not rely on an old local bylaw alone. Municipal materials may lag state law or be under revision. Ask the building or planning department how current rules are being administered, and keep copies of the sources and dates you reviewed.

PERMIT PATH

A practical sequence for early planning

  1. Confirm the use. Determine whether the proposal fits the protected-use definition.
  2. Check local administration. Verify zoning district, site plan review, dimensional rules, fees, and application materials.
  3. Review site systems. Confirm sewer or septic implications, water, utilities, access, and site constraints.
  4. Develop permit-ready plans. Engage qualified design, engineering, septic, or surveying help as the property requires.
  5. Apply and respond. Submit the applicable local permits and resolve review comments.
  6. Build and inspect. Complete required inspections and obtain final approvals before occupancy.

SEPTIC & UTILITIES

Title 5 can be a gating question.

MassDEP guidance explains how Title 5 applies when an ADU increases design flow on a property served by an on-site septic system. An upgrade may be required depending on the existing system and proposed use. Local health rules above the state baseline must satisfy the reasonableness test in 760 CMR 71.00.

For sewered properties, confirm capacity and connection requirements. In every case, review water service, electrical service, heating and cooling, drainage, and construction access early enough to influence scope and budget.

COST & PLANNING

Budget from constraints, not a single price-per-square-foot.

Construction type, existing conditions, foundations, utilities, septic, grading, finishes, professional services, municipal fees, and contingency all matter. Interior conversions can reduce some site work but may introduce structural, fire-separation, moisture, egress, and mechanical challenges. Detached ADUs add site and utility work but can offer more independence.

Review cost drivers

SOURCES & REVIEW

Primary sources used

Last reviewed: September 21, 2026

Educational information only; not legal, zoning, design, engineering, health, septic, tax, or financial advice. Rules and interpretations change. Verify current requirements with official sources and qualified professionals for the specific property.