1. The plot you have taken possession of does not belong to you since it is not the plot no. 24 which you have actually purchased.
2. Plot no.24 has been taken possession of by some one else wherein he has already constructed a house.
3. Find out what is the no. of the plot which you have actually taken possession of.
4. Best option for you shall be to ask your seller to register a rectification deed changing the plot no. replacing the plot no. 24 with the no. of the plot you have actually taken possession of (if he is the owner of both the plots).
5. If he is not the owner of the plot you have taken actual possession of, then you have been cheated by the seller.
6. Lodge a police complaint against him for cheating you. File a Writ Petition before the high court against police inaction if the police refuses to register a FIR based on your complaint.
7. You can also file a Recovery Suit claiming recovery of the entire amount you have paid to the seller with interest, damage and cost.
8. If you construct your house on the plot which you have not purchased but given possession of, then you are exposed to long lasting litigation from the actual buyer of the said plot.