Everything homeowners ask about building an ADU in Boston — answered with 2026 data.
Yes, ADUs are allowed in Interior ADUs as-of-right in owner-occupied 1-3 family homes citywide; detached/attached by ZBA variance (by-right only in Mattapan) in Boston, MA. The maximum size for a detached ADU is Not as-of-right — ZBA variance required (by-right only in Mattapan). Attached ADUs can be up to Not as-of-right — ZBA variance required (by-right only in Mattapan). Boston has an ADU-friendliness score of 5/10. 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 Not as-of-right — ZBA variance required (by-right only in Mattapan). Attached ADUs are limited to Not as-of-right — ZBA variance required (by-right only in Mattapan). Junior ADUs (JADUs) have a cap of Interior ADU (the as-of-right type): up to 900 sq ft and no more than 50% of living area, within existing footprint. Maximum height is 35 feet. You are allowed up to 1 interior ADU per lot (Mattapan may add a detached unit) on a qualifying lot.
Rear setback: 20 feet (applies to variance/Mattapan detached units). Side setback: 5 feet (applies to variance/Mattapan detached units). Front setback: 20 feet (applies to variance/Mattapan detached units). Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
None required for an interior ADU. Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
Required (owner-occupied 1-3 family home; condos and LLCs ineligible). 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. Plan check: 4-6 weeks. Pre-approved plans availability: Not available.
Average ADU rental income in Boston: 1-bedroom units rent for $1,800-$2,400/month, 2-bedroom units for $2,200-$3,200/month. Short-term rentals: Not permitted. Expected ROI: 5-7% annual return.
The minimum lot size for an ADU in Boston is No minimum for an interior ADU. 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.