Basically, the rules for educational institutes (all kinds – buxibans, kindergartens, high schools, everything) can be quite elaborate (maximum X students per class, teachers must have certain credentials, must obtain separate permission to hold classes on different subjects…), so of course many of them try to use loopholes like having two businesses next door to each other and presenting them to the public (and staff) as one business but to the authorities as two. When push comes to shove, for example, a foreign teacher with a work permit for one side of the building caught working on the other side can end up getting deported, and the owner can be fined.
For some of these rules, the rationale is fire safety, or I suppose now also covid safety. For others, it’s to keep quality standards up (which can also have the opposite effect). In any case, study the rules carefully.
Detailed discussions can be found in old threads about kindergartens.