How Much Does a 900 Sq Ft ADU Cost in Massachusetts?

A Practical Cost Guide for Greater Boston Homeowners

If you're a homeowner weighing an accessory dwelling unit (ADU), the 900-square-foot size is likely on your mind for a reason: it's the largest unit most Massachusetts homeowners can build by-right under the state's Affordable Homes Act. Naturally, the first question is what it costs to build one.

Short answer: a 900 sq ft ADU cost in Massachusetts generally falls between about $200,000 and $360,000 for a detached, stick-built unit, which works out to roughly $300–$400 per square foot. Where a specific project lands within that range depends on the ADU type, your lot's site conditions, access, and the level of finish. A converted basement or garage of similar size typically costs less; a brand-new detached cottage sits at the higher end because it includes its own foundation, structure, and utility connections.

Below, we break down what actually goes into that number, why 900 square feet is a meaningful threshold in Massachusetts, and how to think about cost per square foot when you're planning at the top of the by-right size limit.

Key takeaways

• A detached, stick-built 900 sq ft ADU generally costs about $200,000 - $360,000 in Greater Boston; attached units and conversions typically cost less.
• Cost per square foot for a detached unit at this size runs roughly $300–$400, and often falls below the rate of a smaller unit.
• 900 square feet is the maximum size an ADU can be while remaining buildable by-right in most Massachusetts single-family districts (or 50% of the main home, whichever is smaller).
• Boston is excluded from the statewide by-right law and follows its own ordinance.
• The biggest cost drivers are the ADU type, your lot's site and access conditions, and utility connections, not the square footage alone.

Quick cost and type summary for a 900 sq ft ADU in Massachusetts