Before or at the beginning of your rental, your landlord must also help you: the terms of the contract must be fair and legal for both parties. Good question, Robin. When a lease expires, the tenant usually moves. If the relationship continues without a new lease or extension, then tenants and landlords operate with a monthly agreement. I would recommend checking the language of the initial lease on the terms if the residency continues after expiration. I would also suggest going to the local housing authority to find out more about the monthly contractual terms in that state. To answer your question, if he moves in without permission, you would be violating the agreement and the terms of your lease. A lease (also known as a rental agreement) is a legally binding contract and a serious matter. .