This is something I’ve been wondering. Say you have an apartment building that is several decades old, would it be better sustainability wise to try and make the old building greener or tear it down and rebuild it? Tearing it down and rebuilding would require a lot more resources and potentially energy than retrofitting, but a new building would have better insulation whereas an old building would likely be straight brick or concrete, more efficient HVAC systems, and usually have units that are physically smaller so more units and therefore people would be able to fit in the same size building (though that last point is definitely more due to capitalism than environmentalism, due to rising real estate prices). I also imagine you can’t really retrofit more advanced green features like grey water recycling, advanced HVAC systems, etc without so much effort that it would approximately equal the resource consumption of rebuilding, I’d love to be wrong though. Does anyone have any experience or research on which option would be better?