When you arrive in Leuven you are obliged to register your stay at the city hall. This is a general rule by the Belgian law, even for short stays.
If you rent a place inleuven, unless you provide your landlord with the correct reogistration papers, a city tax may be applicable and your landlord is allowed to charge it from you.
This can easily be avoided by registring correctly and providing the correct papers.
If you are a student at KUL, they will provide you with all necessary information and papers to register correctly. If you still need help, have questions or are not a student, you can contact your landlord.
You can find all information about the city registration procedure in the following pages :
City tax on second place of staying: