In the impact summary, I changed the functional unit to ‘m2 GFA,’ and it only seems to to divide the total impact by my primary function GFA.
For example, a design with three functions; Residential 100m2 GFA, and Office 50m2 GFA, Hotel 50m2 GFA. If the total GWP is 1000 kgCO2, and I set the functional unit to ‘m2 GFA,’ then the impact summary will give a total impact as 100 kgCO2/m2 GFA. With a total GFA of 200m2 it should return 50 kgCO2/m2 GFA.