While the addition is under construction, don’t add the energy or Gross Floor Area (GFA) of the addition to your property. The energy use of the space under construction is not going to be the same as that of the real property use, so you don’t want to judge the energy use of your school/hospital/office/etc. on the energy use of the construction.
Once the addition is complete, and the space begins use as it is intended (as a school/hospital/office/etc), then you should add the new square footage into your Property GFA, and start including its energy use in your metrics.
This FAQ has details about how to update your GFA once your addition is complete.