Most lenders allow you the facility to overpay your mortgage, normally by 10% of the amount owed each year (subject to their own terms). However, if you either overpay beyond this amount or repay the amount in full whilst within an initial product period (a 2 year fixed rate for example), there may be a penalty to redeem the mortgage amount. This is normally a percentage of the amount you owe, and will normally be anything between 1-5% of the balance.