Mortgages are normally secured on the same property, using a legal procedure known as 'first charge'.
This means that in the eventuality of the borrower defaulting on payments, the bank has a right to repossess and sell the property, and their debt is the first to be settled from the proceeds. If there is any money left, this is usually given to the borrower providing there is not a 'second charge' on the property.
A second charge may be in place if the borrower had raised further loans, secured on the same property. This would be a normal event, if there was a new loan for house improvements.