Any back taxes against the property have to be paid and brought current. If you've added on to your home, you'll probably need a certificate of occupancy to reflect the addition. Most likely, you'll need a survey to show the bank or finance company the legal description of the parcel. You'll probably need your income tax returns for the past two or three years to show the bank your income. You'll have to sign a promissory note and a mortgage or deed of trust to secure the bank or finance company.
Many banks prepare second mortgage documents on their own, using forms approved by their counsel.
In one case, a leading commercial bank in New York City made a loan to a customer in Connecticut that was intended to be a first mortgage on his residence. The employee preparing the loan document picked up the wrong forms, using those for a second mortgage. So be certain what it is that you sign. Generally speaking, second mortgages are much easier to qualify for. You may be able to refinance an existing loan for less than the cost of a second mortgage.