Modelling Existing Heating Plant serving new Extension

Hi all, I'm looking for some guidance on modelling a heating system for a new school extension that will be served from the existing boiler system. One of the options for a new school extension is to utilize the existing building's heating plant which has sufficient capacity, rather than installing a separate heating system for the extension. To what extent does LEED require me to model the existing heating plant? Do I have to create a model of the existing school and populate it with all related inputs such as constructions, occupancy, internal gains, schedules etc? Then, when I run the simulations for LEED, I exclude all of the existing building's energy except the energy relating to the heating plant and add this to the extension's total energy consumption? What are the baseline requirements? Your help is greatly appreciated. Thanks Dónal