I am a landlord. My tenant says he has the right to have a guest in his apartment. He says the guest could stay as long as he wants and he does not have to pay. He also gave this guest a key. What is the law on this situation. Am I able to charge additional rent for this person?

What does your written rental agreement or lease say about it, I would assume you have a written agreement that limits guests to a number of nights without being added as a resident?

I don't see how you would be able to charge additional rent. Does your rental rate go up based on the number of tenants in an apartment, if not why would you be able to charge extra for someone else moving in?

As chuck said, what is your policy as stated in your lease?