You need a couple more pictures from different perspectives, plus a few more plans - such as side elevation, first floor plan, second floor plan & roof plan. Right now it is a bit hard to decipher by an outsider.
Anyway, if this is a mistake, it is a big mistake. In such a case, they can't force you to buy without fixing it. But it might not be a mistake, just need more information to tell. Why not ask the builder to explain it to you?
