Hi, Check the Title of the flat is clear or not. You don't need to pay Service Tax and VAT. Builders charge because it is thier regular course of business of building and selling so they charge Taxes. (but not of that Lady) Registration charges should be paid by you. After Registration is over. check the doucments online on Registration website showing details of Flat and Sale Deed Number. Is in your name and details of the flat are updated on the website. (Encumbrance Certificate EC is record of registered documents over a property check on website of Registration and Stamps Department).
I think Rakshit is right. You need not pay VAT & Service Tax as the same is applicable only when you purchase flat from the builder. But in your case, mother is gifting property to her son.