House renovation permission

You have the only option to contest the complaint and proceed as per law on the subject through a local prudent lawyer.

BMC may compound and allow, if it is within permissible limits.

There are thousands of such buildings/apartments, and depending on the space allowed, BMC may not take the complaint seriously.  You may receive a formal notice from BMC to satisfy the complainant.  BMC may not dare to dismantle the structures and if there is a Building regularization Scheme in the future you can get the same regularized after paying a nominal penalty.  There may also be exemptions for getting approvals for less than normal building construction for less than 100 yards plot.  If there is such a danger of demolition in the future, contact a local advocate and get an injunction.  If the construction is as per laid down norms and no violations as per standards, the BMC may levy a nominal penalty for not having such approvals.  Meet the ward councilor and request him for help.

Apply for permission for renovation first, if denied , then get it legitimize or regularize the renovation  by paying their deemed penalty.

The renovation could be unauthorized, but not illegal. In my understanding, Law allows such construction to be regularised on payment of fine. Please verify.