Wow, thanks for all the responses... I checked on here yesterday and the mod hadn't posted this thread just yet. Sorry if I'm behind...
Sourgirl - I've been with him for about a year and started staying in his place in June. This is a house he and his gf moved into about 2 1/2 years ago. They've been together for about 5-6 years. He never really thought about marrying her since she doesn't want to get married. I've never really had a panic attack when he's wanted to go be with her, in fact I've only ever encouraged him to but pretty much since I moved in he hasn't. The only time I've talked about a panic attack to him was when she's appeared in my space in middle of the night unannounced. The episode before this I would roll over to find her just standing there or wake up to her sleeping on the far side of the bed against the wall. I've had other panic attacks but they were usually related to things going on outside of the relationship (ie, work, sibling drama, etc).
Redpepper - Yes I've told her this in a face to face discussion, through email, even my fiance and approached her with it. I completely agree with how you think. I was asked to stay there by my fiance, we've established months ago that the basement was to be my area... especially at night. I've told her more recently that any issues with him should be discussed with him. I wrote what I felt was a very friendly email explaining why I have an issue with this, she just got very angry and apparently deleted the email after only reading a few sentences. She did go back and read it later, but things have been strained since then. I'm not sure how much clearer I can be to her.
Nycindie - kinda wish I could, although the laundry room is down there and since this is a rental I don't think we can...
AnnabelMore - He has told me he's talked to her and asked her not to come down while we're sleeping. They've had extensive conversations that ends up her getting pissed and making him feel bad for speaking up...or at least that's what he tells me. I'm not around during these conversations. Knowing the way they communicate it was probably all done through email. He sees this whole thing as a problem because I'm having issues with it, he's really ok with her being down there... I'm not. So far he's respecting that tho and hasn't pushed it but has tried to support me and respect my boundaries.
As far as I know they haven't really talked about why he won't sleep in her room. Though he has told me some reasons... she asks him to and deflects the request or comes up other possibly truthful reasons. He has taken to cuddling with her in her room when I'm not around, but I don't think they are really sexual with each other. I think for him their relationship has devolved but she's looking the other way by insisting that everythings fine and he's not directly talking about it.
Casey - As far as "mine" vs "theirs"... I think it depends on the individual and the others involved to determine if it will be more communal or not. In my particular instance I feel I'm ok with most things being communal, I just need a place (even if it's one end of the basement) that is my own and not invaded by those uninvited. It's a place I can go to chill out away from others, I need downtime... others, perhaps like yourself?, may not need that. Glad you gained a new perspective