We had our adult daughter living with us for a while when she was in grad school. Our house happens to have a bedroom with private full bath and closet, plus separate entrance, over the garage. It also has access to under the eves storage space and we have a basement, so room to store her furniture. That arrangement worked reasonably well for us. She had no kitchen, but wasn't home for many meals, anyway. Now my husband has that room for his office at home, and he loves it. I wanted it for a sewing room, but....that will never happen!
Best of luck with your house hunt. You are wise to start early.