Charge them for that roommate (also get them added onto the lease). I charge a $200 shared bedroom charge each month for additional roommates (college town where rent is very competitive). This is normal for student oriented housing where we charge by the bedroom. I would definitely go for some type of rental increase and state that the person must be on the lease in order to occupy the dwelling. Your house your rules! I'd also suggest adding some type of language in your lease about adding additional tenants or roommates and that adjustments to the rent may be made if they do that. If everything went south and you got taken to court without it in your lease, a judge probably won't find in your favor. It might be best to raise it at the end of the lease term and not directly correlate it to the new roommate or give any reason for a rate adjustment other than "market rates rising." Again most of this depends on the lease they have signed. If it explicitly states two parties' names on the lease only, then absolutely they are adding someone else, so really a new lease entirely could be in order and it's at your discretion to offer them one. as NoNonenseLandlord says, if they choose not to comply, kindly ask them to leave!