In the US we mainly just have fire codes, usually set locally, which dictate the number of people that can be in a building, but this is more about how fast the building can be evacuated in the case of an emergency than how much space each person has to work in. It's possible some local zoning laws dictate the density of the office, but again that's mainly about traffic and parking, not about workers' wellbeing.

Microsoft used to classify a- contractors as one half head, v- as one quarter head, to get around occupancy requirements

No, no, it's because contractors being contractors, they aren't in the office half the time. And vendors are in the office even less.

Though try working as a contractor at Microsoft without your ass in that chair eight hours/day and see what happens.