OP you're asking if you're unreasonable, and when people say yes, you're sniping at them. Why ask if your not prepared to hear the answer?
I agree with all the previous posters that it's not fair for a 13yo girl to share a room with a 3yo, unless in a situation of serious poverty where it can't be helped, which based on your responses, isn't your situation.
Other posters have given their experiences of growing up with the same setup and resenting their mum. If I were your daughter I would resent it hugely, and want to leave home a.s.a.p. when I turn 18. Especially if you're as critical of her in person as you seem to be on here. Wanting your own space at 13 is far from "spoiled", it's very unfair to view her concerns like that.
I would do anything to ensure my 13yo daughter didn't have to share a room with a sister that much younger than her through her teenage years, and that includes all the suggestions people have made here - having the 3yo in with you on a permanent basis in the bigger room (including her clothes etc) so the 13yo can have the small room to herself, setting up a bed in the living room for yourself (could put a divan type single with a proper matress which has a throw and cushions on in the day so it looks like a day bed), cut out unnecessary spending (of which you seem to have a lot) to save for an extension or to move house. Etc etc. (I don't see why you're tied to this bungalow and can't move house under any circumstances). The list is quite endless.
Bottom line is you sound like you think your daughter should be grateful just for being born and for you providing her with food and clothes etc, all of which are very basic requirements as a parent. I agree with PP that despite all this "making memories" nonsense, her overriding memory of her teenage years will be having to share a room with her tiny sister and having a mum who didn't understand or listen to her, and the pain that caused, rather than a few trips to Spain. But why even ask when you clearly don't want to hear that?