From my perspective, and I don't have step-children, but have and had friends that are. Giving the daughter no space clearly says to her she isn't wanted. (Not that it's what you mean, just how a teenage would interpret it.) There's not even a guest room that she can say is just hers when she's there if the room is a playroom. And if the room was set aside for her, it's like you're kicking her out, even though she doesn't live with you and doesn't visit much. That being said, I had friends whose parents kicked them out at 18, and that was just what they did. There wasn't any love lost, and no ill intentions, it was helping them grow up, and it's what worked for them. (But they also knew it was coming for several years before it was done.) I also think something to take into consideration is your relationship, or what you want your relationship to be with your step-daughter. If you want to be the "step-mom" forever, then I doubt it matters. But if you want to be another "mother," then you need to make sure she always feels welcome in your home, and her not having a room for herself could send the wrong message.