EC should be allocated on a project-by-project basis (treating the reuse or reclaiming of the materials as separate from the original construction project) in order to highlight the benefits of retrofit and reuse projects. However flexible a building may be for reuse, there is still the upfront carbon cost that should be accounted for, especially if the end-of-life treatment is still unknown - what if the building gets demolished anyway? I know Larry Strain has studied and written about retrofit and material reuse, mostly focused on improving existing buildings. Here’s his paper:
http://carbonleadershipforum.org/2017/02/09/the-time-value-of-carbon/