How to Choose an ADU Builder in Massachusetts

A Practical, Honest Guide for Greater Boston Homeowners

Choosing who builds your accessory dwelling unit (ADU) is the decision that shapes cost, timeline, and whether the project gets permitted at all. So it’s worth slowing down before you sign anything.

Short answer: To choose an ADU builder in Massachusetts, start with feasibility rather than a sales pitch, then verify three things before anything else, a valid Massachusetts home improvement contractor registration, a construction supervisor license, and current insurance. From there, weigh how fluent the builder is in your town’s zoning, how transparent their pricing is, and how honestly they answer the questions below. The right builder tells you when an ADU isn’t the right call, not just when it is.

Demand for ADUs has increased, and so has the number of builders entering the space. The state projects 8,000 - 10,000 ADUs to be built over five years under the new law. There were 844 ADU applications filed in the first six months of 2025, with more than 550 approved by July 2025. That growth is exactly why it pays to vet a builder carefully. The rest of this guide walks through how, and the specific questions to ask an ADU contractor before you commit.

The key steps to choosing an ADU builder in Massachusetts

1. Start with feasibility, not a sales pitch

The best ADU projects begin with a clear  answer to one question: is an ADU actually buildable on this lot? This is where theory and real-world conditions can dffer. A plan that looks fine on paper can run into setbacks, lot coverage limits, septic or sewer constraints, or access problems that only surface once someone studies the specific property.

A builder worth hiring will want to understand your lot before quoting a price. Horizon ADU’s $200 feasibility report, tells you what’s realistically buildable on your property before you invest in design. If a builder is ready to promise a finished unit before anyone has looked at your setbacks or site conditions, treat that as a signal to slow down.

2. Verify licensing, registration, and insurance first

Before you evaluate anything subjective, style, confirm the fundamentals. In Massachusetts, a contractor performing residential work on an existing one-to-four-unit owner-occupied property must be a registered Home Improvement Contractor (HIC), and structural work requires a licensed Construction Supervisor. You can verify a contractor’s HIC registration and any disciplinary history directly through the state’s Home Improvement Contractor lookup on Mass.gov.

At minimum, confirm the builder holds:

• A valid Massachusetts Home Improvement Contractor (HIC) registration.
• A Construction Supervisor License (CSL) covering the type of work your ADU involves.
• Current general liability insurance and workers’ compensation coverage.

For reference, Horizon ADU builds under an unrestricted construction supervisors license and a home improvement contractor license, and we’re fully insured. Don’t take a contractor's word for it, ask for their license numbers and verify yourself.

3. Look for genuine local zoning and permitting fluency

An ADU builder can be technically skilled and still get you stuck at the permitting stage if they don’t know Massachusetts and Greater Boston zoning in detail. This is often what separates a smooth project from a stalled one.

Under the Affordable Homes Act, ADUs under 900 square feet (or 50% of the principal dwelling’s gross floor area, whichever is smaller) are buildable by-right in single-family zoning districts across most of the state, no special permit or variance for a single ADU, and owner-occupancy requirements are prohibited. The by-right provisions became effective on February 2, 2025. But there are critical local nuances a strong builder will raise unprompted:

• Boston is excluded from the statewide by-right law and has its own ADU ordinance. A builder who applies the statewide rules to a Boston property without checking the city ordinance doesn’t know the local landscape well enough.
• Rules still vary significantly by municipality: size caps, setbacks, height, lot coverage, and design review differ town to town. A plan that’s compliant on paper still has to be buildable on your specific lot.
• The paperwork doesn’t stop at zoning. A code-compliant ADU still needs a building permit, compliance with the Massachusetts State Building Code (780 CMR) and the state ADU regulations (760 CMR 71.00), Title 5 septic or sewer compliance, and a Certificate of Occupancy before anyone can legally live in or rent the unit.

You can read more in our overview of Boston and Massachusetts ADU regulations. The point when interviewing a builder is simpler: they should be able to talk about your town’s rules specifically, not just the state law in general.

4. Understand how they build: stick-built vs. modular

How a builder constructs an ADU affects cost, customization, and whether the unit even fits your lot. Broadly, ADUs are either stick-built on-site or manufactured in a factory and craned into place. At first glance, modular can look faster. But the tradeoffs matter, especially on Greater Boston’s tight, irregular, and sloped lots: