The lender covers the full 100% value of the property plus lends an extra 25% of the properties value on top. The additional 25% loan above the property value may be used for a variety of purposes, such as home improvements, furniture, consolidate debts, stamp duty or to pay the solicitor.
125% mortgages are made up of a maximum of 95% secured against the property and up to 30% as an unsecured loan but with the same mortgage interest rate applied across both parts.
When taking a 125% mortgage you must be aware that you are instantly encountering a negative equity situation because you are borrowing more on the mortgage than the property value, however if property prices increase you will then have equity in the property.