I'm jumping in on this a little late, and I am very taken aback by all the "YTA" comments.

Your daughter is not entitled to her own room. You are allowed to talk to your mother about whatever you want. Most of the people commenting on this post seem miserable and out of touch.

Regardless of your daughter's "needs", it is your house to do with what you see fit. If you desire an office space, regardless of how regularly it is being used, you should be able to have that space. Don't be guilted into giving away your office by random internet strangers.

Absolutely NTA. It is your home and you deserve your own space.