This mortgage assumption agreement allows a new borrower ("Borrower") to assume an existing mortgage from the original borrower. A mortgage assumption is the process of a buyer taking over, or assuming, the seller's existing home mortgage.Requirements – NJSA A. 1. A cover sheet (or electronic synopsis). A loan assumption is a lending process where a borrower formally takes over or "assumes" a current home loan from a home seller. Thinking of taking on a mortgage from a family member? Then you'll likely look into a mortgage assumption agreement. How to fill out the Mortgage Assumption Agreement Document? The revised Form MT15, Mortgage Recording Tax Return, is now available.