Ah, her birthday's in April......
I'd remind her that the flat is out and tell her you're not going to discuss any birthday plans for a month as it's too early just now anyway. By that time, she'll hopefully probably have changed her mind anyway.
Around the time of my DD's 16th birthday - a few months ago - she took to frequently saying that she could do whatever she liked when she was 16. It took a lot of effort for me to avoid saying things like, 'But we're still in charge of the household/You might think that, but how will you fund plan X? etc" and I was in a cold sweat thinking I might have no control.
It was all bluster though....
I realised that my DD still wants parental approval and does value my opinion on lots of things BUT she also wants, as Flow4 puts it, increasing amounts of control, or freedom to make her own choices as I put it. The difficulty is that her choices sometimes don't meet with my approval.
Your DD either secretly doesn't want to have an all weekend party really, even though she'd like to be confident to enjoy the idea of that and her objections are really to her own confused feelings about it/growing up OR she'd love to do that even though it's a scary idea as well as an exciting one but ONLY if she thinks she's not displeasing you.
She just needs some time and space without the distraction of arguing about it and this will blow over.