If you are looking to rent out the property you own with your ex they will probably want you to change the mortage to a buy to let. That way the mortgage will be assessed on the income from the rent on the property, leaving you to get an ordinary mortgage based on your salary for the new property. You will still have to meet all criteria on both mortgages, so there will have to be enough equity in the old property and you will have to have a deposit for the new one. How much will depend on the Mortgage Companies you go to. Ball park figure they will expect the rental income to be 130% of the monthly interest. This allows for the interest to be covered plus any other costs like management fees to let the property.