An Assumption of Existing Mortgage Addendum is an agreement that allows a buyer to assume the seller’s existing loan in a real estate transaction. This addendum outlines the terms of the loan assumption and must be signed by both the buyer and the seller. It is usually used when the seller has an existing loan on the property and the buyer wants to take over the payments instead of getting a new loan. There are two types of Assumption of Existing Mortgage Addendum: voluntary and involuntary. In voluntary assumption, both parties — the buyer and the seller – agree to the terms of the loan assumption. In involuntary assumption, the lender agrees to allow the buyer to assume the loan, but the seller does not have to sign the addendum.