Everything homeowners ask about building an ADU in Boston — answered with 2026 data.
Yes, ADUs are allowed in Select residential zones in Boston, MA. The maximum size for a detached ADU is 900 sq ft or 50% of living area (whichever is less). Attached ADUs can be up to 900 sq ft or 50% of living area (whichever is less). Boston has an ADU-friendliness score of 6/10. Check our full Boston ADU rules page for complete zoning details.
ADU construction in Boston typically costs $200,000-$400,000. The cost per square foot averages $220-$440. Garage conversions are more affordable at $100,000-$220,000, while prefab ADUs cost $150,000-$320,000. Permit fees add $4,500-$7,500 to your budget. See our detailed Boston ADU cost guide.
Detached ADUs in Boston can be up to 900 sq ft or 50% of living area (whichever is less). Attached ADUs are limited to 900 sq ft or 50% of living area (whichever is less). Junior ADUs (JADUs) have a cap of Not allowed. Maximum height is 35 feet. You are allowed up to 1 ADU per lot on a qualifying lot.
Rear setback: 20 feet. Side setback: 5 feet. Front setback: 20 feet. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
1 space per unit. Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
May be required in some zones. Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Boston is typically 90-150 days. The plan check process takes approximately 4-6 weeks. Pre-approved plans availability: Not available.
Average ADU rental income in Boston: 1-bedroom units rent for $1,800-$2,400/mo/month, 2-bedroom units for $2,200-$3,200/mo/month. Short-term rentals: Not permitted. Expected ROI: 5-7% annual return.
The minimum lot size for an ADU in Boston is 5,000 sq ft. This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Boston, a garage conversion typically costs $100,000-$220,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.