When you overpay your mortgage, you may want the flexibility of getting your money back if you need it (subject to eligibility). Borrowing back allows you to do this. We hold all overpayments in an overpayment reserve, so if you want to withdraw money you've previously overpaid - you can.
If you borrow back your overpayments your mortgage balance and monthly payments will increase.
Nationwide stopped offering the borrow back feature on new mortgages taken out on or after 4 March 2010.