Disagree with Henrique on this one (but that might be my purist LCA for design attitude where I’m just wanting to always reward good design). If this strategy of having shared facilities between floors with stairs is a net benefit I think it should be reflected in the LCA. Some other things to consider:
– Do the floor voids mean that services (toilets and kitchens / dining areas) are smaller per floor because they’re shared between floors?
– Is there a reduction in lift energy due to people using the stairs
– What is the reduction in functionality (eg NLA or GFA) and is it justified?

Providing these are all considered I think applying it in the LCA is valid and I’d encourage it.