I have a a single story building that requires a Baseline system 6 (per table G3.1.1A), which according to G3.1.1 is modeled on a floor by floor basis - So I have 1 system. Within this, I have an unfinished fallow space @ 480sqm. Following G3.1.8, G3.1.10(c) and G3.1.10(d) as no heating and cooling system exists, the heating and cooling system in the baseline and proposed case should be the same. 

In selecting this system, should I use a new and separate System 6 (as the overall building type meets this definition) for both proposed and baseline models or should I use a System 4? (as this specific thermal block is less than 2300sqm per table G3.1.1A) and similar single zone exceptions dictate the use of System 4 (e.g. G3.1.1 Exception b)

Thanks in advance. 

Graham