In some cases you can sell part of your land when you have a mortgage on the property but before you can sell part of your land you have to first get the approval of the lender and the lender of the mortgage may insist that a professional valuation be carried out in which you'll be required to pay for.
And then you'll also need sufficient equity in the land or home to support your mortgage if you do get approval from the lender to sell part of your land.