Jesce,
Yet another topic that needs standardization, especially in North America. Yes - it can result in big differences for EC intensity. It’s another reason why having more metrics beyond total kgco2/m2 can be really useful.
RICS has their own system which is based on the International Property Measurement Standard.. It’s worth checking out. The IPMS is huge and sort of hard to interpret. RICS spells it out more concisely, which is nice.
Ultimately it all depends on what LCA requirement or purpose of assessment you’re pursuing. They will likely have different definitions. My opinion is that whether it’s part of the GFA or not it’s still carbon emitted, so we need to work towards a system that captures as much scope as possible and reports it in clear and useful ways. But we’re not there yet… We’ll be getting into this a bit for the CLF benchmarking study.