Firstly just because you pay assessment tax and repair cess for the structures does not mean that they become legal
Property tax is also payable for illegal structures
Thus merely paying taxes does not mean that the structures become legal
The BMC has issued you notice to show documentary evidence showing that the structures are legal
You need to reply to this notice by saying that because of the inaction of BMC itself you are not able to produce the documents and plans as required since its BMC's own case that the plans were eaten by white ants and that they dont have any records
So if the authority itself does not have the original plans then if the owner has himself applied for certified copy of plans, how can the BMC expect the owner to produce the plans
Thus the notice itself is bad in law and the demand of 30 lacs is illegal