Thinking of taking on a mortgage from a family member? Then you'll likely look into a mortgage assumption agreement.Mortgage assumptions where a buyer assumes the existing mortgage loan of a seller have fluctuated in popularity since the 1980s. Requirements – NJSA A. 1. A cover sheet (or electronic synopsis). How to fill out the Mortgage Assumption Agreement Document? An assumable mortgage allows you to transfer your current mortgage to your buyer, potentially allowing them to take advantage of your favorable interest rate. How long does it take to close?