ADU Costs in Boston, MA
Estimate ADU costs in Boston, Massachusetts based on local size limits, allowed types, and zoning rules. Plan your budget with real data.
Boston's Local ADU Ordinance
Unlike most Massachusetts municipalities, Boston operates under its own zoning charter with ADU-specific rules that may differ from the state baseline. This means some state-level rights may not apply in the same way. Check the specific rules below for Boston's requirements.
Cost by ADU Type in Boston
Interior conversions are the most affordable option at $100-$200 per square foot, since you're reusing existing structure. Attached ADUs typically cost $150-$250 per square foot, sharing a wall and utility connections with your existing home. Detached ADUs are the most expensive at $200-$350 per square foot, but offer the most privacy and design flexibility.
Budget for a Full-Size ADU
With 1250 sq ft allowed in Boston, a maximum-size detached ADU could cost $250k-$438k for construction alone. Add 15-25% for permits, design, site work, and utility connections. A smaller footprint can significantly reduce costs.
Return on Investment
ADUs in Boston can generate significant rental income while also increasing your property value. With a population of 675,647, Boston has strong rental demand that supports competitive rates. Since owner occupancy is required, your ADU is best viewed as a long-term family investment that also generates income.
Important: State Baseline Data
The information on this page reflects Massachusetts state baseline rules. Boston may have additional local requirements. We recommend confirming with your local building department before starting your project.
Boston ADU Rules at a Glance
Max Size
1250 sq ft
% of Home
75%
Max Height
25 ft
Max Stories
2
Front Setback
25 ft
Side Setback
5 ft
Rear Setback
5 ft
Parking
1 space(s)
Approval
BY RIGHT
Owner Occupancy
Required
Rental Allowed
Yes
Short-Term Rental
Not allowed
Boston Building Department
Boston Inspectional Services Department
1010 Massachusetts Ave # 5, Boston, MA 02118, USA
Phone: (617) 635-5300
Frequently Asked Questions
How much does it cost to build an ADU in Boston, MA?
ADU costs in Boston typically range from $150,000 to $350,000+ depending on the type. With a maximum size of 1250 sq ft allowed, a full detached ADU (the most expensive option) in Boston could cost $200-$350/sq ft. Interior conversions are typically the most affordable at $100-$200/sq ft.
What is the cheapest type of ADU to build in Boston?
In Boston, a conversion ADU is typically the most affordable option. Interior conversions reuse your existing structure (basement, attic, or garage), eliminating foundation and framing costs. Boston allows detached, attached, conversion ADUs, giving you flexibility to choose based on your budget.
Are there ADU financing options in Boston?
Common ADU financing options available to Boston homeowners include home equity loans (HELOCs), construction loans, cash-out refinancing, and renovation loans (FHA 203k or Fannie Mae HomeStyle). Some Massachusetts programs also offer ADU-specific financing or incentives.
What rental income can I expect from an ADU in Boston?
ADU rental income in Boston depends on size, finishes, and location. A full 1250 sq ft unit (2-bedroom) can command premium rents. With a population of 675,647, Boston has strong rental demand that supports competitive rates.
Do ADU permit fees add significantly to costs in Boston?
Permit fees in Boston typically range from $1,000 to $5,000 depending on the project scope. While not the biggest cost factor, budget for building permits, plan review fees, utility connection fees, and any impact fees. By-right approval in Boston means you avoid the additional costs of hiring a lawyer for public hearings.
How accurate is the ADU data for Boston?
The information on this page reflects Massachusetts state baseline rules under the Affordable Homes Act. Boston may have additional local requirements that differ from the state baseline. We recommend confirming with your local building department before starting your project.
Where can I find all ADU regulations for Boston?
Visit our complete ADU rules page for Boston, MA for a full overview of zoning, permits, size limits, setbacks, parking, and costs. We cover all Massachusetts municipalities with data sourced from local ordinances and state law.
Check Your Address
See exactly what ADU rules apply to your specific property in Boston.