If you have to re-plat, you can't do this: re-plating would reduce the amount of acreage providing security on the mortgage, but you cannot reduce the lender's security without their consent or agreement, since you have no power to unilaterally (without their consent) alter the agreement (the mortgage) between you and them.You can build a home partially on the land they have a security interest in, but bear in mind that if you default, they will get that portion (if any) of the home on the land on which they can foreclose. However, their having a security interest does not bar you from using or improving, including building on, that land, so long as you do not impair their security.