Do I need a partial release or something else?

[Lauren Manning]

Bought a home on 2 lots. Before closing, we were informed by the city that the person that had the house under contract before us (deal fell through) used a lawyer to have the lots split and the addresses updated/ changed. Our home and the extra lot are listed on the deed and under one mortgage. We want to build a home on the other lot to live in - not to sell. We would rent out the current home. Since we aren’t selling, do we still need a partial release? Having trouble finding an answer to our situation. Thanks!