Can i add another floor in the house which is in mother in laws name?

As the property stands in the name of your mother in law, you have to prove that you have spent some money in the construction of the same whereby you can claim interest in the property in question. This is your matrimonial home and you cannot be thrown out without due process of law.

In law if you can prove that you and your husband had spent some money on the construction and it can be proved that it is a matrimonial asset then you can assert some right.

You are currently having the possession of the property and unless someone proves a superior title, you cannot be threatened for leaving the house.