Regulations and laws vary considerably between jurisdictions both in content and extent and in enforcement.
The most common regulations B&Bs must follow pertain to safety. They are usually required by local and national ordinances to have fire resistance, a sufficient fire escape plan in place, and smoke detectors in each guest room. Kitchens and equipment used to serve meals are also often required to be monitored for hygienic operation, but there are significant national and local differences.
In Hawaii, it is illegal to open a new bed & breakfast on Oahu as of 1989. The reason for the moratorium is to force home owners with extra room to rent out their extra space to low income residents who otherwise cannot afford housing on crowded Oahu.