The people in charge of enforcing the rules historically have trouble proving its happening (even if they know). No trouble at all. Even if the address is not

The people in charge of enforcing the rules historically have trouble proving its happening (even if they know). No trouble at all. Even if the address is not published (which it usually isn't) you can search for a property by map. There were court cases here in which the landlord sued the tenant. The tenant claiming that he only rented out two or three times, which was belied by the reviews on the prperty, which proved without a doubt that he rented out at least 20 times. The court awarded all illicit gains from the rentals to the landlord. Also, be prepared to be kicked out of your appartment pretty swiftly if you rent it out without the explicit permission of the landlord. Permission is hardly ever granted, because it usually severily (and rightfully) pisses off the other long term tenants in a house.