Want to extend house with no blueprints or plans

I have a very old house that belongs to my grandfather. They want me to extend it, however when I looked into the documents, there was no plan of the house or any blueprints since it is really old but is definitely strong. Now the only document that exists is a WILL wherein my maternal grandfather's mom has divided the property between the 3 sons and my grandfather built the house in his share of property.

Now if i need to extend it and make another floor. Can you advise me as to is it possible to get the plans sanctioned by the municipality now. We have paid house tax all this while and it is up to date. Moreover, is there a way to find out if the base is strong enough?