Exclusion Rules and Co-Signer Liability
The DTI exclusion and your liability on the loan are two different questions.
Fannie Mae’s rule addresses how a lender calculates your debt-to-income ratio. It does not address, and does not change, your legal responsibility as a co-signer.
What the exclusion requires
Fannie Mae’s Selling Guide allows a lender to exclude a monthly debt payment from the borrower’s DTI calculation when the lender documents, with the payer’s most recent 12 months of canceled checks or bank statements, that someone else has been making the payments without delinquency (Fannie Mae Selling Guide B3-6-05, dated Aug. 5, 2026).
Who can be the payer
Fannie Mae’s rule does not require the person making the payments to also be named on the loan. It does require that the payer not be an interested party to the property transaction, such as the seller or the listing or selling agent.
What a missed payment changes
The documentation has to show payments without delinquency. A payment history with missed or late payments does not meet the standard, and a lender would calculate the debt into DTI in the usual way.
Why co-signing still carries risk
The CFPB notes that co-signing a loan means agreeing to be responsible for the debt if the primary borrower cannot pay, and that missed payments can affect the co-signer’s credit (CFPB, last reviewed Sep. 12, 2023). Excluding the payment from a mortgage DTI calculation does not release a co-signer from the underlying car loan.