Extension Risk in UK Seasoned Interest-Only Mortgages

Past-term interest-only (PTIO) mortgages are interest-only (IO) loans that have reached contractual maturity without repaying the principal balance, leaving borrowers to resolve repayment after the due date. These loans are gaining prominence in the UK as legacy IO cohorts—particularly those originated prior to the global financial crisis (GFC)—approach maturity.

Within UK residential mortgage-backed securities (RMBS), seasoned IO loans represent approximately 4% of total outstanding collateral balance and roughly 75% of seasoned nonconforming collateral, aggregating around GBP18 billion, according to Intex. While a portion of these loans redeem before or at maturity through remortgage or property sale, pre-GFC vintages exhibit lower redemption rates at maturity, increasing the likelihood of loans transitioning to PTIO status. As a result, a meaningful share of these loans remains contractually past term but often continues to make payments, creating a maturity and extension dynamic…