In an effort to encourage more apartment and condo development, then-governor Charlie Baker in 2021 signed what is known as the MBTA Communities Act, the most dramatic effort in more than 50 years to address Massachusetts’ housing crisis.

The law requires 177 cities and towns in the MBTA’s service area to pass new zoning to permit multifamily housing units in dense areas, largely around transit stations. The mandate does not require the units to be built, but calls for new zoning to allow them by right.

“Multifamily housing” is defined under the law as three or more housing units.

Guidelines for enforcing the law divide communities into four tiers with different levels of obligation, based, with some exceptions, on the size of the community and the level of MBTA service it enjoys.

Rapid transit community — served by the Red, Blue, Orange, Green, or Silver lines, carries the greatest obligation under the law; must open its zoning to permit a minimum number of multifamily housing units equal to 25 percent of the community’s current year-round housing stock. Deadline for rezoning is the end of 2023.

Commuter rail community — served by the MBTA commuter rail; must rezone to permit a minimum number of multifamily housing units equal to 15 percent of the current housing stock. Deadline for rezoning is the end of 2024.

Adjacent community — must rezone to permit a minimum number of multifamily housing units equal to 10 percent of the current housing stock. Deadline for rezoning is the end of 2024.

Adjacent small town — carries the lightest obligation under the law; must rezone to permit a minimum number of multifamily housing units equal to 5 percent of the current housing stock. Deadline for rezoning is the end of 2025.