consequence of constructing on land in possession

relative may not say anything but in future his heirs may object it.
because after your relative his heirs will become owner of land.
so before building any construction you should get his permission in written which would be conclusive proof for the future.
else your investment may b waste

,

respected sir, considering heirs not going to involve in this case and paying money to relative & getting something is written not a viable option, what is the legal maintanbility of any compliant/suit to authorities /police by third parties ? i also wish to pay consulting fee if any of respected experts wish to guide on legal side on this matter.