We've done a lot of renovations on our 50+ year old house since we moved in more than 20 years ago. When we created a basement room for the girls a few years ago, we envisioned them hanging out with friends doing whatever teen girls do.
Instead, it's their studio apartment. The table we used for puzzle early in the pandemic is now their school/craft space. The couch is permanently in double-bed mode. The girls lounge on it during school, to play video games and watch movies. They have snacks and drinks in the pantry.
Their friends said it looks like a page from an Ikea catalogue. I take this as a huge compliment.
If they had a basement bathroom, they'd never have to come upstairs.
I often think about how much harder it would be not to have the basement space. The girls would have their bedrooms, but it's really nice for them to have two spaces to call their own. During a time when we really don't leave the house much, the more options the better.