A mortgage in principle is an agreed figure from the lender that they would be willing to lend you. In order to do so they will gather basic information and perform a background credit search. A mortgage in principle is not a guarantee and a full application and assessment will have to be made before the lender can issue you with a mortgage offer. A mortgage in principle can last between 60-90 days depending on the lender. Because a credit search is needed, multiple decision in principles could have a negative effect on your credit score.