You could approach your current lender to renegotiate the terms and obtain further funding on the site, this might involve a remortgage.
Or you could approach a new lender and remortgage all properties.
Or you could approach your current lender, find out how much they need to release the site as security and then go to a new lender to obtain a mortgage with one of the conditions being that this amount be paid out from first stage payment to clear old loan, then further stage payments to build.
All entirely possible if your income will support further borrowings and the value of the site/or house when built will support the loan.
Suggest you post the value of your properties and loans and your income and a broker could comment.