What if you found a house that had a ladder up to an attic, and you filmed your actors going up into it. Then you could use a different location, one that gave you the freedom to design the set to your liking, and it could create the illusion of an attic room?
Years ago, I had a bit part in a movie, and they filmed at a huge house, that was suppose to be a lake side house, with a boat dock out back. The house we filmed at wasn't even close to a lake. Then we went down to a lake, and filmed on a local boat dock. Then again to a 3rd location, this time it was the big boat, that was suppose to be tied up at the dock, behind the huge house.
Once it was all edited together it flowed like it was all one location.
Just because you need a large attic room, doesn't mean you have to film in a large attic room. If you have one friend that has a cool entrance to their attic, but the room is too small, use the entrance, and film the scene someplace else. You mention wanting a window. Is there a shot from outside of your actors looking in? Could even use another location that has an exterior that matches more your vision.
That's the magic of film making, doesn't have to be all one location, but you can make it look that way. Of course, finding a singular place would be nice, but not always a requirement.
Jeff