Private accommodation is based on mutual voluntary agreement. Of course, arrangements can be made with your guest regarding a contribution to housing and energy costs. However, the amount of this compensation should not exceed the housing component (€80.91 per month). Be cautious when formalizing the accommodation into, for example, a rental agreement. A rental contract introduces additional rights and obligations for both parties. Additionally, a rental agreement can have financially disadvantageous consequences for you and your guest. Make sure to gather information about the pros and cons beforehand. A good starting point is this page.