You can add one or more new buildings from the site level. Each building has its own set of Model options and is simulated separately but shares site Model data with the other buildings on the site.
When you select the Add new building option you are presented with the Add new building dialog which allows you to make some basic settings:
When you press OK to close the dialog the building is created with the various selected settings and in most cases starts you in Draw block mode so you can start adding geometry straight away. If you are not ready to do this simply press the <Esc> key or right click and select Cancel.
See also the Creating a model Tutorial