The LEED guidance on baselines is very high-level. From our experience, as long as the baseline is a fair point of comparison for the building, it can be used. Examples of this include a similar building that serves the same function—like an office building. For the projects that were non-standard building types, like an airport terminal, we have seen teams use their first design as the baseline, evaluate that, then suggest changes to reduce impacts to meet the 10% reduction target in GWP and two other categories. The modifications that you may need to consider (if any) when looking at a real-world building to serve as the reference building are discussed in detail here.
For Whole-Building LCA, how do you choose the baseline building?