I updated my signature as you mentioned, Solver. Thanks. Also I found the tutorial on plot plans so I have my lot properly defined.
I've gone through the links and searched the forum and this seems to be the best approach:
Create a template first with defaults and preferences set to your liking.
Use the template to create a plan of the existing lot and existing structures (for example MyHouse_Existing.plan). You could skip this version of the file and go straight to 3 below, but this gives you a good reset point to start from if you need it. Storage is cheap..
Copy this plan to one called MyHouse_Complete.plan. This will have all 3 structures, the existing house, the addition, and the separate garage. It will serve as the master plan but not as the materials list.
Copy the master plan (once done) to MHouse_Addition.plan and delete the original house (as much as possible). This will allow for the creation of the materials list. Alternatively, you can split it into separate ones for the addition and the garage.
I've seen several posts asking this question or something similar. It sounds like it would be a good idea to have a way to identify items that are existing (unless there is and I haven't found it yet). That way you could easily design an existing structure (exclude it from the materials list) and then add structure to it.
Does this make sense ?