Which mortgage has the highest interest rate? And will both mortgages allow you to make a one-off reduction without paying a penalty. It normally makes sense to reduce the debts which are charging the highest interest rate first. if they're both on a short term period, there's a possibility that the first mortgage you have to renegotiate will end up with a higher interest rate as inflation is starting to soar, which means mortgage interest rates will increase. In that case, reducing the bigger mortgage would make more sense as overall that will be the one which costs you more interest in the longer term.