the flat i'm staying in was bought under my parent's name. Now my parent wants to go China for good. where can I find Notary service provider to witness / document my parent's consent to sell the flat in future? how much is the average charge? Thanks in advance.

A better option would be for you to be a borrower for your parents’ housing loan without having to add your name as your parents’ property. Be that as it may, though you might wish to either depose the property before your intended purchase of the HDB BTO.