Can I buy a house with a mortgage in Israel. Am I not simply borrowing on interest, for personal reasons and therefore not entitled to the heter iska ? Or perhaps there is a heter because of the mitzva of yishuv haaretz ?? This is a genuine question because we are thinking of buying, and we don’t have all the money we need.

If the bank has a Heter Iskah you can rely on it, which is a common practice by orthodox Jews. You can add on the mortgage document that it is subject to the Heter Iskah.