I'd like to be able to say there's a very simple answer, but unfortunately, there isn't because it really depends on the building itself and what its condition is. If there's been a ton of deferred maintenance, and maybe there are requirements for upgraded seismic resistance, there may be no choice but to demolish the building and start over again. I think, from an overall sustainability point of view, it really behooves us to look and see if the building has “good bones”, in other words a good structure that we can save, and we can rehabilitate that building.
The answer is that very many factors must be taken into consideration—economics, safety, health and the availability of technologies to rehabilitate that building.