This is a tough one, but you're paying for half the room, so you should have full reign to decide with whom you live if you have a preference. If you are assuming that your current roommate still wants to live with you next year, you're going to have to try especially hard to break it to them easy.
Why would this be beneficial to you, and could it end up being beneficial for them as well? Is there a certain building or place off campus that you wanted to live in that you know your current roommate doesn't like? Try to be political. Think of some pros and cons to bring up when you tell them your decision.
It would probably be better to talk about this kind of thing face-to-face, like at lunch or at a neutral location like Starbucks or Muddy Waters on Main Street, so that you can show that you have respect for them to take the time out of your day to discuss the issue. Show that you care about their feelings, and maybe even strike up a compromise: Maybe you could be suitemates instead? Or choose to live nearby one another off campus?
Either way, it is better to have the conversation sooner than later, since on-campus housing applications are due soon and they might want some time to find someone as a replacement.
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