FAQ: MBTA Communities Act

The MBTA Communities Act was adopted in January 2021, as part of legislation to strengthen the state’s economy.  It was passed by broad bipartisan majorities in the legislature — the Senate voted unanimously in favor of the Act, and the House voted 143 in favor to 4 against.  Governor Baker signed the Act into law on January 14, 2021. 

The Act, commonly referred to as “Section 3A,” requires 177 Cities and Towns to establish “at least 1 district of reasonable size in which multi-family housing is permitted as of right.”

Said housing must be suitable for families with children, and may not impose age restrictions within the district. (Source)

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The reasoning behind the law is that the greater Boston region, for some time now, has been experiencing a housing shortage. The State’s Future of Work study estimated a shortage of up to 200,000 housing units. Municipalities play a key role through zoning and permitting in determining whether or not housing is built.

To relieve the state’s housing shortage, the law requires communities to amend their zoning and permitting to encourage greater housing production.

Additionally, the state notes that:

• Before COVID19, Massachusetts faced a core challenge in creating enough housing to support young families, workers, and an aging population. The pandemic has further highlighted these needs.

• Massachusetts has among the highest, and fastest growing, home prices and rents of any state in the nation.

• Rising costs have dramatically increased financial pressures on low- and middle-income families, forcing them to sacrifice other priorities to pay housing costs. High housing costs are a primary driver of homelessness.

• The lack of housing production is an impediment to community and economic development.

• These high costs are a disadvantage as we compete economically against peer states. The high cost of living risks future job growth moving outside Massachusetts.

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Under Massachusetts law, multifamily housing means a building with three or more dwelling units, or two or more buildings on the same lot, with more than one unit each.

That can sound like big apartment complexes—but it doesn’t have to be. In fact, many types of small-scale multifamily homes already exist in New England towns like ours. Think triplexes, townhouses, duplexes, or a pair of two-family homes on one lot.

These kinds of housing types are often called the “missing middle”—because they fall between single-family homes and large apartment buildings. They’ve historically been part of small-town New England, but in recent decades they’ve become less common in new development due to zoning restrictions.

Here’s an image that shows examples of missing middle housing types.

The MBTA Communities Act encourages towns to make room for these more modest, neighborhood-scale housing options—not massive developments—by updating zoning to allow them “by right” in at least one district.

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Because Carlisle shares a border with towns served by the MBTA, it is classified as an "Adjacent Small Town" that must zone for 95 units (5% of our 2020 housing stock) of multifamily housing at a minimum of 15 units per acre. See this 2024 overview prepared by our town planner Julie Mercier.

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Very. Carlisle doesn’t have any realistic option to opt out.

As of this writing 129 MBTA communities are in compliance, with 20+ more actively working toward it. The Massachusetts Supreme Judicial Court has upheld the law as constitutional, and the Attorney General’s office has shown it’s serious about enforcement. In fact, the state successfully sued the Town of Milton for failing to comply.

Non-compliance risks more than just losing state grants. The Attorney General has made it clear that the state could sue or appoint someone to rewrite our zoning for us.

See the Planning Board’s Mission Statement as it relates to the mandate.

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Yes—and we’ve known that for a long time.

More than 90% of Carlisle’s housing stock is made up of single-family homes. Today, over 20% of homeowners—and nearly 70% of renters—are considered cost-burdened (meaning they spend more than 30% of their income on housing). These aren’t abstract numbers—they represent our friends and neighbors who struggle to stay in the town they love. They may represent you.

Carlisle has talked about this lack of housing diversity for decades. We’ve produced housing production plans. We’ve created an Affordable Housing Trust. We’ve set housing goals in our Master Plan. We’ve studied the Highland Building and the cranberry bog building as possible locations for workforce housing.

This is more than a state mandate—it’s also a chance to address some of our own long-standing housing challenges in a thoughtful, locally driven way.

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No. The MBTA Communities Act does not require any of the zoning to include deed-restricted or income-limited units as “Affordable Housing.”

That said, towns can choose to include a requirement that up to 10% of multifamily units be affordable to households earning 80% or less of the Area Median Income (AMI). For Carlisle, the AMI is ~$161,000 (so 80% of that is ~$129,000 or less). That income range includes many essential workers—like teachers, first responders, and Town employees—and others who contribute meaningfully to our community but may be priced out of it.

In short, affordability isn’t required by the Act, but we may have the option to include it—if it fits Carlisle’s goals, selected zoning approach, and financial feasibility.

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No. Carlisle will be considered in compliance once we adopt zoning that meets the state’s requirements. Section 3A is a zoning mandate, not a construction mandate.

The law requires us to create area(s) where multi-family housing is allowed by right—95 total units at a minimum density of 15 units per acre, without age restrictions. But it does not require the Town (or anyone else) to actually build anything.

Instead, it creates capacity: private property owners would have the option—not the obligation—to build multi-family housing in the new zoning area, provided they meet all local regulations. The state won’t step in to construct anything, and Carlisle won’t be forced to build housing.

In short: the MBTA Communities Act is about zoning, not construction. It’s about creating the possibility, then letting the market decide what happens from there.

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This is the primary tool Planning Boards across the state—including Carlisle’s—are using to comply with the MBTA Communities Act. It’s a way to add capacity without eliminating existing zoning rights.

An overlay district is a special zoning tool that adds a new set of rules on top of existing zoning. It doesn’t replace the current zoning—it simply provides an additional option.

In a residential neighborhood currently zoned for single-family homes, an overlay district might allow multi-family housing as an added possibility. Property owners could still build a single-family home under the original rules—but now they’d also be allowed to pursue the new option, as long as it meets the overlay’s requirements.

Think of it like adding a transparent layer to a map: the base zoning remains in place, but the overlay introduces new possibilities.

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Not if we want it to count toward compliance.

Section 3A sets clear rules about what types of land can and can’t count toward a town’s multifamily zoning obligation. The law requires zoning to apply to buildable, developable land—meaning it must be realistic, not just theoretical.

The following types of land are excluded from counting toward compliance:

• Wetlands

• Land under Conservation Restrictions (CRs)

• Bodies of water

• Roads and easements

• Public land that is not already designated for housing, unless the town provides a detailed development plan showing how that land will be used for housing

In other words, towns can’t zone for housing on land where housing can’t legally or practically be built. Public land might count, but only if the town goes a step further and proves that it will be used for housing—not just zoned for it.

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By planning with care—on our terms.

Yes, 15 units per acre sounds dense, but it’s a misleading phrase without context. Carlisle has significant control over how we meet the mandate—and we can do so in ways that still feel like Carlisle.

The Planning Board is actively exploring a range of thoughtful zoning approaches, including scattered small-scale overlays, Residential Open Space Communities (ROSC) that cluster homes and preserve open land, and higher-density developments on smaller footprints.

One important lever we have is pace. We can influence how quickly development happens through which zoning approach we take. This matters a lot when it comes to things like managing impacts on schools, roads, water, and town services.

The law sets the framework. We shape the outcome.

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The planning board is currently analyzing four distinct approaches. Due to how the ‘development areas’ would be calculated combined with limitations created by Title V septic requirements, all approaches meet the required 15 units per acre minimum density threshold even though it may not look or feel like it.

I created a visual explainer to help make the Planning Board’s work easier to follow—it’s not an official town document. I’ve tried to be accurate, but if something doesn’t match what’s on the Planning Board website, that’s on me. I’ll update it as needed and always welcome corrections.

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This is a critical issue—we all rely on Carlisle’s shared groundwater, and any new development has to take that seriously. The Planning Board has been working with the Board of Health to understand the risks, constraints, and opportunities.

It’s important to note that Section 3A does not require Carlisle to build or pay for any infrastructure, water-related or otherwise.

I’m still getting up to speed on all the details, but here’s the basic picture (you may want to have the Visual Explainer handy):

• Approaches 1 and 2 (which involve dispersed or clustered zoning strategies) would rely on existing Title V regulations, which cap development at 4 bedrooms per acre. That means a 2-acre lot (Approach 1) could support up to 8 bedrooms—what is already allowed for single-family homes today. Using the same Title V formula of 4 bedrooms per acre, Approach 2 (on 15-acre sites) could support up to 60 bedrooms in 31-32 units.

• Approaches 3 and 4 (the higher density strategies) would require different water and wastewater solutions—most likely a private wastewater treatment facility on site and/or a deal negotiated by a developer to access water/sewer infrastructure from a neighboring town.

There’s a lot to keep learning about water resource management—from nitrogen loading to drawdown impacts on private wells—and I’m committed to listening to experts, asking good questions, and making sure we fully understand the factors involved in protecting our water resources.

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It’s true that for some zoning approaches—particularly smaller-scale, distributed ones like Options 1 and 2—Title 5 will continue to limit development based on septic capacity. In those cases, bedroom count are effectively capped.

But other options, especially the fourth one that concentrates all 95 units on a single site, carry more risk. That’s because they rely heavily on the assumption that Title 5 will hold. And that’s an assumption we absolutely need to test.

We’ve seen in communities like Lexington and Bedford that developers pursue the maximum allowed by zoning, regardless of what the town intended. If Carlisle zones for 95 units in one location, that zoning signals what’s legally permitted. A developer could look at that and say, “If I solve the infrastructure challenge, I can build all of this.”

And now, with the state’s new $50 million Momentum Fund—designed specifically to make projects like this viable—a developer could secure funding to install an onsite wastewater treatment facility. That would override Title 5, and the bedroom limits would no longer apply.

Bottom line:
What I want to avoid is a scenario where voters support a single-site option in good faith, believing it’s the safest path—only to be shocked and upset when a developer builds exactly what they feared. That’s not just a bad outcome—it’s a failure of the public process. And we can prevent it by having the full, fact-based conversation now.

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I understand why that fear resonates with some people, but it may be based on a misunderstanding of how zoning changes happen.

The good news is, zoning doesn’t change automatically. Any future changes would still need to be proposed, publicly vetted, and approved by Town Meeting—just like this one. Nothing in the MBTA Communities Act takes away Carlisle’s control over future zoning decisions.

For those pointing to Bedford or Lexington with concern: it’s not that developers are exceeding what those towns allowed. It’s that they built exactly what the zoning permitted—because those towns underestimated what was possible under their own rules, and badly underestimated how quickly it would happen. Those are mistakes we can avoid.

If we zone thoughtfully—with clear dimensional limits, caps on building size, open space protections, and site plan review—we can guide where and how development happens. But not all approaches give us the same ability to manage the outcome. A single-site strategy, in particular, leaves fewer tools to shape what gets built—and may invite exactly the kind of large-scale development some residents are hoping to avoid.

No—MBTA zoning does not increase a developer’s ability to pursue a 40B project. Chapter 40B already allows developers to bypass local zoning if they include at least 25% affordable units and the town hasn’t met the state’s 10% affordable housing threshold. Carlisle hasn’t met that threshold, so 40B remains an option regardless of how we zone for MBTA compliance.

What MBTA zoning does is create a by-right path for multifamily housing—but under local rules that we define, with constraints we can design to reflect Carlisle’s character and priorities. That gives the town more control than a 40B project typically would, but it does not eliminate the 40B risk unless and until we reach the 10% affordability threshold.

This also opens up an important community conversation: How important is Affordable Housing to us? (Defined as housing for households earning up to 80% of Area Median Income—in Carlisle’s case, up to $132,300 for a family of four.) Some approaches to MBTA compliance will make it easier to include deed-restricted Affordable Housing and help move us closer to that 10% goal. Others may not. That’s a key tradeoff we should evaluate together, based on facts—not fear.

The Finance Committee (FinCom) is currently conducting (or will undertake) a fiscal analysis of the different zoning approaches Carlisle is considering. That analysis will help us better understand the potential costs and benefits of various full build-out scenarios.

That said, many communities that have studied this question have found that multifamily housing can have a net positive fiscal impact, especially when the homes are modestly sized and thoughtfully located. Of course, those outcomes depend on many factors—unit size, number of bedrooms, pace of development, and more—so it’s not guaranteed that Carlisle would see the same results. But the trend in other towns suggests that multifamily housing doesn’t automatically mean a negative impact on town finances.

What’s in a fiscal impact study? These studies typically estimate how much new housing might generate in property tax revenue, and compare that to the cost of providing town services like schools, police and fire, water, and infrastructure maintenance. They often model different scenarios—varying unit sizes, numbers of bedrooms, and household types—to get a sense of the likely range of impacts.

If you’re curious how other towns have approached this, the fiscal impact studies linked below are worth a look. While these places are different from Carlisle in size and structure, their analyses can help us think through our own planning.

• Presentation by Massachusetts Taxpayers Foundation

• Town of Milton

• Town of Mansfield

• Town of Medfield

• Town of West Newbury

• Town of Weston

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Our schools are one of Carlisle’s crown jewels, and protecting their quality matters to all of us.

Adopting a compliant zoning plan doesn’t trigger new construction—it just makes it possible. Impacts to school enrollment will only occur if and when multifamily housing is built, and even then, the impacts depend on:

• Which zoning approach is chosen

• The type and size of units (number of bedrooms is more relevant than total units)

• How quickly development occurs

We can’t limit bedroom counts, but we can shape what’s likely by setting square footage maximums and making informed predictions about what developers would do. For example, Option 4 (a single high-density site) poses the greatest risk of rapid enrollment impacts, since it’s most likely to be built all at once.

—> Want a preview of what we might expect? Check out the fiscal impact studies linked in the previous question. They use a range of multipliers, and you might be surprised by how few students are projected from smaller units.

It’s also worth considering a broader trend: Carlisle’s population has grown, but our school enrollment has dropped. From 2010 to 2023:

• Population rose by 340

• Enrollment fell by 87 students

That trend—more residents, fewer school-aged kids—raises important questions about long-term enrollment and community sustainability. See the trends on DataTown →

In any case, Carlisle needs a thoughtful, data-driven analysis of future enrollment impacts—which will depend on what gets built and how. FinCom and the School Committee will be key partners in modeling those scenarios.

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Carlisle is one of 35 “Adjacent Small Towns” subject to the MBTA Communities Act. While each town is unique, it can be helpful to see what our peers are doing as we shape our own approach.

Most towns with our December 2025 deadline haven’t voted on a zoning plan yet—they’re in “interim compliance,” just like we are. Many are aiming to bring proposals to their Fall 2025 Town Meetings.

Below are links to ten “Adjacent Small Towns” with housing stock similar in size to Carlisle, so you can explore their websites and see how they’re approaching compliance.

From what I’ve seen so far, Carlisle is one of the most rigorous and creative in exploring multiple pathways to meet the mandate—carefully weighing tradeoffs, consulting with other boards, and engaging the public. That’s something we can be proud of.

• Plympton

• Ashby

• Princeton

• Sherborn

• Essex

• Nahant

• Paxton

• West Newbury

• Dover

• Rochester

• Harvard

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The Planning Board is tracking public questions and comments, and you can follow that evolving Q&A in this live document (updated after each meeting).