You can draw the boundary of your site easily on Snaptrude.

First go to the 2D view of the first storey. Then, choose the “Draw” tool (L) from the Design toolbar & select “Space” as the object type. Begin drawing a closed loop of the boundary on the design canvas. Lastly, label the space as “Site” by double-clicking on the mass. This will change the material, thickness & level of the mass in a site plan. If you have a CAD drawing or an image, you can import them as a reference & trace it out from there as well. (See how to import CAD or Image.)