Also keep in mind that the building system is a database. It considers something a new “base” if you set down a foundation/pillar/fence foundation that isn’t snapped to anything else. Thus, if you have a cluster of detached buildings all in the same area, each would have its own record in the building database, and therefore each would have its own first block.
If you are an admin and link everything across the map with decay runners you later delete? Still considered a single base, and only the origin point of it will ever get attacked – at least as the mechanics currently work.