Illegal construction but Registry done

Hi, I purchased a flat in Indirapuram Ghaziabad. This is a multistory builder flat. When we purchased we simply thought if the registry is done that means it is legal and in registry it clearly specifies the area and design of the flat. We somehow missed on looking at the layout of the building S=since the registry was done, Bank approved the loan by scrutinizing every corner of the flat, etc. Now we got a notice that only 8 flats were approved and the builder constructed 16 flats in that premise. Though the area of the building is intact it was supposed to be 8 flats, not 16. This is just a year old building. Please tell me as a customer what we can do in this case. This is a builder fraud and government ignorance when the registry was done and also, why they didn't check all this before when the builder was actually building 16 flats instead of 8.