ADU Rules in Boston, MA (2026)

Boston, MA permits accessory dwelling units (ADUs) on residential properties. ADUs — also called granny flats, in-law units, backyard cottages, or secondary suites — are secondary housing units on a single-family or multifamily lot. State and local law governs the approval process. In Boston, the Boston Inspectional Services Department (ISD) handles ADU permit applications. Below you will find the current size limits, setbacks, fees, and timeline for building an ADU in Boston.

ADU Size Limits

ADU Size Limits in Boston: Detached ADU: size varies by zoning — see official source (max height: varies — see official source). Attached ADU: size varies by zoning — see official source.

Permit Fees

ADU Permit Fees in Boston: Plan check fee: $500–$3,000. Building permit fee: Based on construction value — typically $2,000–$8,000. Impact fees: Massachusetts municipalities may not charge impact fees beyond standard connection fees; Boston does not have a general impact fee system. Utility connection fees apply.. School fees: No school impact fees in Massachusetts — school funding is through property tax and state aid. Total estimated permit cost: Varies.

Permit Timeline

Boston ADU Permit Timeline: Boston ISD processes ADU building permits — typically 6-12 weeks for plan review. Large or complex projects may require BPDA review. By-right ADUs per state law should not require discretionary approval. Boston's Article 80 review applies to large development — typically not triggered by single ADU projects. Apply through the Boston Inspectional Services Department (ISD) online permit portal. Phone: (617) 635-5300.

Setback Requirements

ADU Setback Requirements in Boston: Front setback: follows zone standard. Side setback: no minimum. Rear setback: no minimum.

Parking Requirements

Parking Requirements: No additional parking required for ADUs within one-half mile of a transit station per Massachusetts Affordable Homes Act (2024). Boston's extensive MBTA subway (T), commuter rail, and bus network means many properties qualify. Boston has reduced parking requirements generally as part of its transit-oriented development goals.

Restrictions & Requirements

• •ADU must comply with Boston Zoning Code dimensional standards for the zone
• •Detached ADU must be located in rear yard — not in front yard setback
• •ADU cannot be sold separately from the principal lot
• •Boston Short-Term Rental regulations strictly limit STR — ADU as primary Airbnb rental is restricted
• •Historic district requirements (Back Bay, Beacon Hill, South End) may restrict exterior modifications
• •ADU must comply with Boston's energy efficiency requirements (Stretch Energy Code)
• •ADU must have permanent foundation — no recreational vehicles or mobile homes

How Many ADUs Are Allowed?

Number of ADUs Allowed in Boston: Single-family lots: 1 ADU per single-family lot by-right per Massachusetts Affordable Homes Act (2024); additional units may be possible through Boston's ADU program or Article 80 review. Multifamily properties: Additional accessory units may be permitted in 2F and 3F zones per zoning and state law provisions. Massachusetts Affordable Homes Act (2024) requires by-right ADU allowance in single-family zones. Boston's local ADU program, launched in 2017, predates state law and may allow additional configurations. The BPDA oversees large residential projects requiring Article 80 review.

Single-Family Lots

1 ADU per single-family lot by-right per Massachusetts Affordable Homes Act (2024); additional units may be possible through Boston's ADU program or Article 80 review

Multifamily Properties

Additional accessory units may be permitted in 2F and 3F zones per zoning and state law provisions

Statewide Preemption

Historic District Considerations

Frequently Asked Questions