Approval in principle (AIP), also called mortgage approval in principle, is an indication from a lender of how much you might be able to borrow based on information such as your income, debts and deposit. It is not a formal mortgage offer or a guarantee that your application will be approved. The lender will usually carry out more detailed checks, including reviewing documents and the property, before making a final decision. An AIP may also expire or need updating, so check its terms and current lending criteria directly with the lender or a regulated mortgage adviser.