An Agreement to Extend Security Interest is a legal document that is used to extend the term of an existing security interest. A security interest is a type of lien that establishes a claim over an asset in order to secure the payment of a debt or other obligation. An Agreement to Extend Security Interest is used to extend the duration of the security interest beyond the original term of the security agreement. The types of Agreement to Extend Security Interest include mortgage extension agreements, automobile loan extension agreements, and equipment loan extension agreements. Mortgage extension agreements are used to extend the term of an existing mortgage. Automobile loan extension agreements are used to extend the term of an existing auto loan. Equipment loan extension agreements are used to extend the term of an existing loan for equipment. All of these agreements must be signed by both the lender and the borrower in order to be valid.