They have a guest room which I use.

My understanding of what I was providing them came largely from what they provide me - the guest room they set up has their art on the walls and a few useful items (a small ladder etc) in the wardrobe, which is about the level of use I imagined.

I imagined some bookshelves and a chair in one corner so I could sit in there and read. That’s all.

I didn’t want to LIVE in it. I’d have a perfectly good bedroom and bathroom of my own. I just want to be able to put some of my more neutral belongings in it, so the space isn’t completely useless to me when they’re not present to use it.

Apparently that was not the agreement. I was explicitly told “it’s not a guest room. We don’t want your art or your books in our space”

…which is quite different to what I understood the situation to be.