Hi Jordan,
It’s not usually the demolition itself that has the high carbon impact; it’s the construction of a new building, with (especially) new structural and new facade materials.
The demolition carbon emissions include:
- emissions from equipment on site (fuel and water for the demolition contractors and their equipment)
- emissions to transport the waste away
- emissions to process the waste (off-site)
As I noted, these are typically quite low. The UK has a guidance document you can access, the RICS Professional Statement on Whole-life carbon Assessment (2017). It gives some high-level guidance on how you can calculate the carbon emissions from demolition (life-cycle stages C1-C4).
If you convert everything to a per square metre basis, and you will build a new, much bigger building in its place, this will make the carbon impact of the demolition look trivial too.
I appreciate that most of my message probably reads like, “demolition doesn’t matter”. That would be a misinterpretation 
It’s simply that demolition is often relatively low carbon, so the carbon-impact of demolition isn’t the story you want to tell. It’s the carbon impact of starting from scratch, which really matters.