Renting a house to a graduate student in my department
At the time they sign the lease there is no conflict of interest. Further, there is no conflict of interest anticipated throughout the duration of the lease. There is of course the possibility that an unforeseen conflict arises, but this is always a possibility. You are putting yourself at an elevated risk of a conflict of interest, so it is probably worth mentioning it to you department chair. If an unforeseen conflict does arise, just let your department chair know and do what ever is needed at that point.