How much board/rent is fair?

Approach your mother as an adult. Tell her about the 3 bedroom to 1 bedroom situation. Propose to divide the bills three ways. 1 part for her, 1 part for her bf and 1 part for you. You may find it easier to rent somewhere else and stop all this housing drama. That would offer you stability in your life. Also, whose house is this; theirs or or yours. Is it a leased house; whose name is on the lease. It seems this is a lot of drama to go through just for a room.