To create a new building you need to update your Building Configuration data to include a new building definition.
The easiest way to do this is to find an XML chunk in your building data file (e.g. BuildingData.xml) that defines a Building Type:
Duplicate (copy and paste) this chunk back in to your XML file, and then edit the values to suit your new building.
Note: You MUST change the building ID to define a new building. This value must be unique across building, occupants, etc.