Solving the MBTA Communities zoning puzzle

What does gross density of 15 dwelling units per acre mean?

IN 2021, the Commonwealth of Massachusetts adopted the MBTA Communities zoning law requiring municipalities served by the MBTA to zone districts “of reasonable size” for multi-family housing at a “gross density” of 15 dwelling units per acre. The law applies to 175 cities and towns. Some may already have zoning that meets the requirements. For most of the municipalities, compliance will involve local legislative votes, at town meetings and city councils.

In the process of local reform, thousands of public officials, constituents, and advocates will have to become familiar with the law’s novel, complicated, and flexible concept of gross density. I am writing this article to give people a head-start in understanding it.

The state’s implementation guidelines explain what cities and towns need to do to come into compliance with the law’s requirements, including for gross density. The guidelines are highly technical. The assignment to revise zoning, though, is not a bureaucratic task. It is a democratic task. Reform will engage city councilors and town meeting members in vote after vote, city by city, and town by town.

In this epic effort, people will grapple with what the law’s gross density requirement means specifically for their communities’ neighborhoods and places. Many are already asking: “What does 15 units per acre look like?” They want to know what kinds of buildings we are talking about.

To represent what parcel-level densities of 15-units-per-acre look like, I could show photos of 15 townhouses on a one-acre lot, or a triple decker apartment building on a 1/5-acre lot, or a 30-unit condo building on a two-acre lot, or a 150-unit apartment complex on a 10-acre parcel. However, the law, as written and as interpreted by the state’s executive branch, is not actually about these densities. Not directly, at least. The term “gross” makes the concept of density more expansive and flexible.

The MBTA Communities zoning law refers not to the densities of individual parcels, but to the densities allowed across entire zoning districts. District-level zoned density is a new concept in zoning, invented in this new law.

The implementation guidelines explain that gross density can be calculated by assessing how many multi-family housing units the zoning allows to be built on every single existing parcel in the district (or districts), and then dividing that count by the total acreage of the district (or districts). This total (“gross”) acreage includes all parts of the district, including both buildable properties and unbuildable areas like public roads or ponds or the land that the public library sits on. According to these rules, the zoning can qualify for compliance if the relevant zoning districts average a “capacity buildout” of 15 units per gross acre, across the whole district.

If there is any land where multi-family housing cannot be built in a district, then the municipality will have to zone at least part of the district for parcel-level densities greater than 15 units per acre – in order for the whole thing to average out to 15. But, importantly, some parcels or sub-districts of qualifying districts can be zoned at lower densities than 15 units per acre. It just has to average out.

In this way, the law’s “15 units per gross acre” can include zoning for high rises and zoning for very low-density townhouses, or anything in between.

The takeaway: Pretty much any type of multi-family density could be a part of local solutions to the puzzle posed by the state’s MBTA Communities law rules. Any multi-family building could fit into the puzzle, depending on how municipalities design the districts.

If a zoning district is drawn to contain only buildable parcels, and no roads, no public buildings in active use, and no waterways, etc., then it is possible that the allowed parcel-level density of 15 units-per-acre could be the same as the district-wide gross density of 15 units-per-acre.

Why not just draw the district around buildable parcels where multi-family housing will be allowed, and leave the roads out of the district boundaries? In some cases, municipalities will. However, the law’s requirements for minimum district size and district contiguity will mean that unbuildable parcels, rivers, and public rights-of-way will sometimes be included within district lines.

The state has recently released a newly developed tool, called a compliance model, for measuring the gross density of multi-family housing allowed in districts, in accordance with the law. Since the tool is new, municipal planners and consultants are only beginning to test different scenarios for implementation. The range of compliant parcel-level zoned densities will likely be wide, but is not yet known.

Meet the Author

Amy Dain

Public policy research consultant, Dain Research, Newton
In running scenarios for implementation, planners will puzzle the required gross density of 15 units per acre together with other technical requirements such as minimum district size and minimum zoning capacity. The law offers significant flexibility for implementation by municipalities, in terms of allowed parcel-level densities and district sizes and district locations as well. It will be interesting to see how each locality solves the puzzle.

Amy Dain is a consultant in public policy research with Dain Research in Newton. Her website is here and she can be found on her Twitter and Instagram at @amydain. Her series of articles is based on a working paper and a blog post she wrote for the Lincoln Institute of Land Policy.