Why Permit Numbers Don't Tell the Full Story: Inside Massachusetts' ADU Consistency Gap

In January, I published an analysis of what 1,224 ADU permits revealed about Massachusetts housing. The data told a clear story: some towns are embracing the new law, others appear to be dragging their feet.

But permit numbers only tell you what happened. They don't tell you why.

Over the past several months, I've been reading local bylaws — line by line — and comparing them to what Chapter 150 and 760 CMR 71.00 actually require. I've reviewed every publicly available Attorney General decision on ADU bylaws issued since the law took effect. And I've built a structured consistency tracker that maps specific provisions in 28 towns against state law.

What I found is that the gap between state law and local implementation is wider than the permit numbers suggest — and more patterned than most people realize.

The permit data has a blind spot

When the state reports that a town approved 18 ADUs, that sounds like the system is working. And for that town, it might be. But the number doesn't tell you whether the town's bylaw also requires a minimum lot size that the state says is prohibited. It doesn't tell you whether the town restricts ADUs to single-family lots when the state allows them on any lot with a principal dwelling. And it doesn't tell you whether a builder trying to put up a detached modular ADU would hit a provision that appears to conflict with state law.

Permit counts measure activity. They don't measure access.

A town can issue permits and have bylaw provisions that discourage or block categories of projects that should be allowed. These aren't hypotheticals — the Attorney General's office has now issued rulings on ADU bylaws in towns including Canton, Leicester, East Bridgewater, Hanson, Wilbraham, Upton, and most recently Southborough, and in every case, provisions were disapproved because they conflicted with state law.

A pattern is emerging in AG decisions

When you read the AG decisions as a collection rather than individually, the same types of provisions get struck down again and again. Here's what keeps appearing:

Minimum lot size requirements. Chapter 150 and the implementing regulations do not allow towns to require a minimum lot size for a protected use ADU. Canton's bylaw included such a requirement. The AG disapproved it in June 2025. EOHLC now specifically references the Canton decision in its FAQ as guidance for other towns.

Single-family-only restrictions. Several towns attempted to limit ADUs to lots containing only single-family dwellings. State law allows protected use ADUs on any lot with a principal dwelling — including two-family and multi-family properties. The AG has disapproved single-family-only language in multiple decisions, including East Bridgewater (April 2025) and Wilbraham (December 2025, Case #11778).

Bedroom limitations. Leicester's bylaw capped ADUs at two bedrooms. The AG disapproved this provision in May 2025, citing G.L. c. 40A § 3's prohibition on regulating the interior area of a single-family residential building. Bedroom caps also function as occupancy restrictions under 760 CMR 71.00.

Special permit requirements. The state law is clear that protected use ADUs are allowed by right, not by special permit. Yet several towns adopted bylaws that routed ADU applications through discretionary special permit processes. Hanson's bylaw, for example, included special permit language that the AG disapproved because it conflicted with the by-right standard.

Parking mandates beyond half-mile. State regulations prohibit requiring parking for ADUs located within half a mile of a public transit station. Wilbraham's bylaw required one parking space for all ADUs regardless of location — the AG disapproved the provision as written.

Restrictions on ADU type. The most recent AG decision, issued February 17, 2026, addressed Southborough's ban on using modular homes as ADUs. The AG struck the provision, finding that the town's pre-existing definition of “mobile homes” was broad enough to disqualify manufactured housing types that are protected under state law. This is significant — it's the first AG decision that directly addresses construction method restrictions, and it signals that towns cannot use definitional games to narrow what types of structures qualify as ADUs.

What this means in practice

Here's where it gets concrete. Say you're a builder looking at a town with an approval rate above 75%. Looks good on paper. But if that town's bylaw still includes a minimum lot size requirement or limits ADUs to single-family lots, you could win approval for a basement conversion and get blocked on the detached backyard cottage that's actually your client's project.

The Pioneer Institute's November 2025 report, “Beyond Legalization,” documented this dynamic well. Its author noted that local zoning and permitting practices continue to discourage ADU development even after statewide legalization, and that per-capita production in Massachusetts remains a fraction of California's rate. One of the builders quoted in the report noted that setback and septic rules are eliminating potential projects, and that permits remain difficult to obtain in some communities.

This matches what I'm seeing in the bylaw data. The friction isn't always visible in the permit numbers because it operates at the discouragement stage — before an application is ever filed. Homeowners who read their town's bylaw, see a provision that appears to restrict their project, and never apply. Builders who know from experience which towns will make the process difficult. These suppressed projects don't show up in any dataset.

The consistency gap is measurable

Across the 28 towns ADU Pulse has profiled so far, I've identified specific provisions and assigned each a confidence tier based on the strength of evidence:

This isn't a score or a ranking. It's a provision-by-provision mapping with citations, evidence basis, and links to source bylaws. Every finding is anchored to specific statutory language, not editorial opinion.

What the data shows is that most towns have at least some provisions that warrant scrutiny. Very few are fully consistent with state law across every dimension — lot requirements, ADU types, site plan review scope, dimensional standards, parking. The towns that have updated their bylaws since the law took effect are generally in better shape, but even some of those updates have been partially disapproved by the AG.

Why this matters now

We're entering year two of statewide ADU legalization. The first wave of AG decisions has established clear precedent on several provision types. Towns that passed ADU bylaws in 2025 and haven't yet updated them may have provisions on their books that have already been disapproved in other communities.

For homeowners, this means your local bylaw may not be the final word on what you can build. The state law overrides provisions that appear inconsistent, whether or not the town has formally updated its code.

For builders, the consistency picture is a market intelligence question. Understanding which towns have clean bylaws versus which have provisions that are likely to be preempted by state law helps you advise clients, allocate resources, and avoid surprises mid-project.

For policymakers, the pattern in AG decisions is a signal. The same types of provisions keep getting struck down. Towns that haven't yet updated their bylaws can use this data to get ahead of the AG review process rather than waiting for a formal disapproval.

And for anyone tracking Massachusetts housing production, the takeaway is this: the 1,224 ADUs approved in year one are a meaningful start, but the consistency gap suggests the actual addressable market for ADU development is significantly larger than current production levels indicate. Cleaning up bylaw inconsistencies won't just be a legal exercise — it should unlock projects that are currently being suppressed.

What we're building

ADU Pulse's consistency tracker now covers 28 Massachusetts towns with structured, provision-by-provision analysis. Each profile includes the specific bylaw language, the relevant state statute or regulation, our confidence assessment, the evidence basis, and source links.

We track AG decisions as they're issued and update affected town profiles. We're adding new towns based on user demand — Salem, Northampton, Canton, Amherst, and Whitman are next in the queue.

If you're a builder, lender, or housing policy professional who would benefit from structured consistency data, I'd like to hear from you. This is the kind of analysis that doesn't exist anywhere else, and I'm building it based on what the market actually needs.