This is obsolete as of Archicad 15. From that version onward, renovation is the way. Read this instead.
What happens when the existing model and drawings are done.
The basic idea is to keep the existing conditions, both the PLN and the layout book, tucked away safely. It is theoretically possible to get existing drawings out of the addition project, but it's more trouble than it's worth. It's important that the existing work be complete at this point.