How does UpZoning Impact Housing Affordability?

Doesn't MBTA Zoning [Upzoning] require "affordable housing"?

Not necessarily. MBTA Zoning requires 10% of units to be affordable for housing in projects of 3 units or more. Zoning allows a developer to "subdivide", parcels of land into smaller parcels, where the number of housing units could be only 2. See below for an example of how a developer put 2 million dollar condos on a quarter acre of land. He was able to do this because the zoning allows "multi-family housing". And the two million dollar condos are attached by a garage, and so they both qualify as "multi-family housing." The economics indicate that you could tear down a $600K house and put up two of these million dollar condos, using MBTA Zoning, and you'd get zero affordable housing and the loss of an existing "naturally affordable" home. Further, because MBTA Zoning is "by right", and acre of land can be split up into 4 quarter acre lots where two million dollar condos on each quarter acre, and 8 million dollar condos will exist. There are dimensional requirements to limit this, but there are many lots on Central Street which appear big enough for two or more projects where 2 million dollar condos are created. So we expect to see a "naturally affordable" housing unit destroyed to get 4 [or more] million dollar condos.
In Acton there are MANY MBTA Zoning parcels of land where this can happen. BEWARE of MBTA Zoning on your town!

Aren't multi-family homes more affordable?

Not necessarily. It depends on "the market". So one could say "probably", but without evidence in a specific town what buildings want to build, we do not know for sure. In Acton, for example, the minimum price of a new "market rate" condo is $700,000. Here is an example of a "multi-family home" in Acton Massachusetts:

It's lovely. But with a million dollar price tag, it's far from affordable, for people making less than $50K/year, which is the dire housing need in Acton.

Because of this, evidence indicates that MBTA Zoning will make neighborhoods less diverse, and will reduce income diversity.