What laws or regulations have caused the people you know the most trouble? I remember some stories in the past about sewer or water regulations causing problems, but not for tiny houses.
On top of the arbitrary square footage requirements that wlesieutre already mentioned, in a lot of cases it boils down to windows per room requirements, as well as the typical <x> requirements per room, where rooms like kitchen and bathroom are the most problematic.
While I'm generally not a big fan of regulations, I get that there are definitely well-intentioned regulations for legitimate problems, such that landlords weren't allowed to cram too many people into too small a space, or demand that too many people share too few kitchens and bathrooms, but for those who choose the path for themselves, those self-same regulations are burdensome.