I've seen people claiming that I"m wrong, and that Dallas doesn't allow more than four unrelated individuals to occupy a single family home.
I did look at the Dallas Code. There's some room for interpretation, which is why I spoke to a Code Enforcement officer. You can get different answers from different Code Enforcement people, so that's no guarantee I'm right. If someone wants to provide me a code citation or some sort of evidence that the City of Dallas considers any group of five or more unrelated adults to be an illegal use in single-family districts, I'll supplement with this information.
I think Dallas nicely illustrates, though, why everyone should be skeptical of sweeping claims about the average occupancy limit in Texas or the nation. It ought to be easy to figure out how many unrelated people can share a house. In practice, ferreting out the answer for even one city can be a time-consuming, tedious exercise. And there might not be a clear answer.
The Austin model is simply to declare, "No more than X unrelated people may occupy a dwelling." I haven't seen many cities follow this model (I've looked at 15 or so carefully). A more common approach for zoning occupancy limits is the Fort Worth model: You provide a definition of "family" (say, "any number of people related by blood or marriage plus up to X number of unrelated peope"). You define "dwelling." And you then restrict your single-family districts to one family per dwelling and one dwelling per lot (two dwellings per lot if you allow duplexes). Under this model, a household with more than X + 1 unrelated people is not a permitted use in a single-family district.
Dallas doesn't follow either the Austin or Fort Worth models, though.
Dallas does have a definition of "family":
FAMILY means individuals living together as a single housekeeping unit in which not more than four individuals are unrelated to the head of the household by blood, marriage, or adoption.
I read that to mean that up to five unrelated people will be treated as a "family." (Or something. The Dallas Code doesn't define "head of household" or define "family" when a head of household is not present.)
But Dallas doesn't define the single-family use in terms of families. Instead, it defines the single-family use to be, "One dwelling unit located on a lot." A "dwelling unit" means:
one or more rooms designed to be a single housekeeping unit to accommodate one family and containing one or more kitchens, one or more bathrooms, and one or more bedrooms.
Note that a dwelling unit merely needs to be designed to accommodate one family; there's no requirement that it be occupied by only one family. None of this prohibits a large group of unrelated people from sharing a dwelling unit.
That's not the end of the inquiry. Rather than limit single-family districts to "families," shared homes could fall within the definition of a zoning use that is specifically prohibited in single-family districts.
Dallas has been waging a battle against small group homes for addicts and alchoholics for years. (I sat through a CLE presentation last year by some Dallas attorneys that could have been entitled, "How to fight the Fair Housing Act and Group Homes at the same time.") Not surprisingly, Dallas has defined a special residential uses called "group homes":
"Facility" is not defined. Is any shared home a "facility"? Unclear. "Board" is not not defined, but "room and board" is usually understood to include "meals." Even if five roommates are sharing their meals every evening, they're not being provided meals by the "facility." Shared student housing thus doesn't appear to me to meet the definition of "group home."
Dallas has other special residential uses. None of these appear to apply to shared student housing for one reason or another, which I won't wade into. Overall, though, there are several close questions. Enough close questions that I'd really be interested in hearing from someone who practices in Dallas or is otherwise familiar with the interpretation of the City's Planning Department.
A simple way to settle this would be to check actual enforcement practice. I know that Dallas has cited group homes. That doesn't mean that Dallas has been going after groups of students, or even considers shared housing a per se violation of the zoning code.