if your agent claimed it is 2400, and you cannot find that extra 500. Your
agent is in trouble for misreprenting you and you can sue your agent for not
doing the job to recover the loss.
If you found extra 500, but it was built with no permission. If the seller
didn't disclose it the extra 500 without permssion, you can back out with
claiming of seller's misinformation.
If your agent informed you is 1900 plus 500 without permssion, you need to
find out where 500 is. If you cannot find it, you can backout the deal. If
you find that extra 500, then it is on borderline now.
First check if your agent misrepresent you or not, then check if the seller
intentionlally not disclose the information. If house is not the same as
your agent told you, the first one to sue to recover the loss is your agent.