Building Addition / DES / existing pumps in original part

I am not too sure how to interpret this situation: This project is an addition (Phase II) to an existing 5 year old building (Phase I). Phase I is LEED certified. It is on a campus with a District Energy System (DES) providing chilled water and steam. The steam heat exchanger, hot water pumps, and chilled water pumps are in Phase I and were sized for Phase II. At the Phase II project boundary is only a chilled water supply and hot water supply. Otherwise, Phase II has its own HVAC systems. As a modeling approach, do I model the Phase II building ONLY using purchased chilled and hot water, or do both the old and new Phases have to be modeled as one, or should I model Phase II only and account for the pump energy? Or something else? ASHRAE Appendix G Table G3.1-2 (b) asserts that the HVAC system must be "entirely" independent to do the addition only. Furthermore, it would be nice to have the LEED review team view on this interpretation before I start. Thanks for your thoughts.