thanks to both for your replies. I'm somewhere in between the two extremes; my compulsive side does start off every plan based on a 4ft by 4ft grid and I manage to keep the framing plan and almost all partitions to that grid. But as soon as doors / windows or bathroom fixtures and kitchen cabinets become involved, the grid becomes mostly pointless. I'll be happy if I can just keep all foundation walls, and framing layout on the grid and let the architectural details fall where they may.

RE: my original question - I must have been dreaming. While it is certainly much easier to plan 3 independent boxes; there is no way that I could justify actually building the home that way. Assuming I move forward with this plan, I will just take it as far as I can (detailed floor plan , schematic frame layout) and then hand it off to a real frame designer to integrate the 3 boxes into a unified whole.