What sort of complexities are introduced when you construct a building complex in phases?

Regarding the architectural layout of the model, it can be difficult to keep track of what is new construction, what is demolished from the original construction and what remains of the original construction. In order to best convey what is exactly going on, the modeler must carefully consider what is happening in each phase. Then model and annotate the information accordingly. When it comes to the actual construction process, it can be even more difficult due to structural members missing during demolition. Often times temporary structures must be put into place to keep the building from collapsing. This can cause great delays if not carefully considered.

What are the principal advantages of using a single building information model of the existing conditions as the foundation for modeling several proposed design alternatives for a portion of the building?

In short, organization is key. Models are complex enough just considering the original layout of the building. If this is not carefully organized in the proper manner, then wires can be easily crossed. In other words, if you have to independent models being worked on separately, then the models may not agree with each other. Then you could run into constructability issues or inconsistencies that lead to larger problems. By having one master model this is no longer an issue. All information is kept the uniform throughout the modeling process.