Resale house - actual construction vs planning approval vs property tax

We are planning to buy a resale independent house in CBE. I have seen ppl going up with actual construction is about 3000 - 4000 sqft but in approval I see that they have obtained merely about 800-1000 sqft. As well cases like construction of additional floors without disclosing it in the planning approval. And I see them paying lesser property tax by showcasing lesser construction area. What steps the buyer need to do to get it rectified before buying it from the seller.