How is the terrace area calculated?

That reads to me as a pure assumption by the OP that the seller would then no longer be interested. Possibly yes as a reaction, since the (also assumed) fraudulent deception would no longer work. But this is purely my assumption, as very little meaningful information has been provided here so far.

You should work on your communication skills and your understanding of the importance of information for your future construction site,

Actually, it always goes like this with the OP. You really don't want such a buyer.

Why haven't you spoken to the seller yet?

Because he will probably refrain from concluding a contract (unless he is in severe financial distress) if this prospective buyer starts communicating with him...