U-shaped building

Great question!  There is not a way to directly specify this.  It’s not really important information for the energy model, but it does mean that some of the area estimations made by OptiMiser will be incorrect.  You will need to perform your own calculation for Wall Area and specify the foundations us using Area and Perimeter, rather than Length and Width.   The default distribution of wall  area across the four facings of the house will also be off, but this very little impact, unless you are making wall or window improvements.  The wall distribution can be set in the Net Areas popup, accessed on the Walls tab.