You need a couple more pictures & plans.

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?