I agree with your assessment. I live in Boston and conducted some research on this same topic a couple of years back.
I took a look at the state sanitary code too. I thought that may be prohibitive, but was surprised to find that it really isn't.
The Massachusetts state sanitary code (410.420 (D) Minimum Square Footage) provides:
(1) Every dwelling unit shall contain at least 150 square feet of habitable floor space for its first occupant, and at least 100 square feet of habitable floor space for each additional occupant.
(2) A rooming unit shall contain a minimum of 100 square feet of habitable floor space when: (a) The unit contains one single room for living and sleeping only; and (b) Is occupied by no more than one person.
(3) In every residence, each room used for sleeping purposes by one occupant shall contain at least 70 square feet of floor space.
(4) In every residence, each room used for sleeping by more than one occupant shall contain at least 50 square feet of floor space for each occupant.
My assumption had been that maximum occupancy requirements would so restrictive as to prevent most families from living with housemates. This was based on a presumed strong, positive correlation between families seeking to live with unrelated housemates to offset their rent and families living in homes with a relatively high number of people per square foot of living area.
Families seeking to offset the cost of living with an unrelated housemates are also probably seeking to offset the cost of living in other ways too, right? One such way would be not splurging on more living space than is necessary. Less space typically equals lower living cost.
But, living in smaller spaces results in a higher person per square foot, placing the occupants closer to the state sanitary code maximum occupancy thresholds.
I still feel these to be reasonable assumptions.
But they aren't actually dependent on the state’s thresholds. So, are most families seeking to add housemates to offset their rent actually near or over the actual state maximum occupant / square foot thresholds?
Some back of the envelope math suggests no. Let's assume the following:
Now, imagine a family of 3 living in an ~750 square foot home. Let's also imagine that home has two bedrooms, each 100 square feet. Based on the minimum square footage requirements under the state sanitary code, that household compliantly support 4 individuals.
If the bedrooms were 150 square feet each - let's assume this home now just has a smaller kitchen and living room to compensate - the house could compliantly support 6 people.
I recommend determining the maximum occupants based on your living space's square footage. Or maybe a friends. You'll likely be surprised how many folks can compliantly (as far as the sanitary code is considered, anyways) live in a given space.
The Tufts web page has a feedback form. I've taken the liberty of reporting the inaccuracy there, with a link to this post.
My experience in some Southern towns is similar: either applied only to districts or zones so not as widely as people mythologize, or misinterpreted as referring to fewer people than the plain text reveals.
As part of the general discourse around cost of living, Julia and I were talking about families sharing housing. This turned into us each writing a post ( mine, hers), but is it actually legal for a family to live with housemates? In the places I've checked it seems like yes.
While zoning is complicated and I'm not a lawyer, it looks to me like people commonly describe the situation as both more restrictive and more clear cut than it really is. For example, Tufts University claims:
As far as I can tell, all three of these are wrong:
Medford: it's common for people to cite a limit of three, but as far as I can tell this is based on a misunderstanding of the definition of a lodger. Medford:
Since a shared house typically does function as single housekeeping unit (things like sharing a kitchen, eating together, no locking bedrooms, a single shared lease, sharing common areas, and generally living together) this is allowed.
Somerville: the restriction was repealed two years ago.
Boston: defines family as "One person or two or more persons related by blood, marriage, adoption, or other analogous family union occupying a dwelling unit and living as a single non-profit housekeeping unit, provided that a group of five or more persons who are enrolled as fulltime, undergraduate students at a post-secondary educational institution shall not be deemed to constitute a family." Then they define a lodging house as "Any dwelling (other than a dormitory, fraternity, sorority house, hotel, motel, or apartment hotel) in which living space, with or without common kitchen facilities, is let to five or more persons, who do not have equal rights to the entire dwelling and who are not living as a single, non-profit housekeeping unit. Board may or may not be provided to such persons. For the purposes of this definition, a family is one person." I read this to say that a group of people (even students) who live as a single housekeeping unit don't make something a lodging house.
This isn't just my reading zoning codes: a similar question came up in Worcester in 2013: City of Worcester v. College Hill Properties. The MA Supreme Judicial Court ruled that the unrelated adults sharing a unit together did not make it a lodging house because they were a single housekeeping unit and rented the whole place.
In other places there may be different restrictions, but everywhere I've looked so far it looks to me like this kind of shared housing, where a group lives together like a family even if they're not actually related, is allowed.