A mortgage in principle is a guesstimate from a lender about how much money they might be willing to lend you. It’s not a guarantee that you’ll get a mortgage, but it can give you a general idea of what you can afford. The reliability of a mortgage in principle depends on the information you give the lender. If you’re honest and accurate, the guesstimate should be fairly accurate. But if you lie or omit information, the guesstimate could be way off.