I think the issue is with prepared food (especially hot food) and I have to imagine that there are licenses, health inspections, etc that restaurants need to have, and if I could start a restaurant without one, then wouldn't everyone?

However I've been doing more research and found this quote:

Ken Sato, principal environmental health inspector for the San Francisco Health Department, says an underground restaurant would only be investigated in case of a complaint.

"We'd issue a notice to cease and desist operation," he says. "

from this article: http://www.metroactive.com/papers/sonoma/04.06.05/dining-051...

So: I guess green light?