Bringing context into the model is one essential step; however, the images and buildings imported cover more than your specific building site.
Sketch the boundary of your site the same way you would sketch the outline of a building, making sure you have the Site tool selected:
With Create items automatically enabled the Site will be created once the boundary is closed.
Otherwise use a tap inside the boundary you’ve drawn to create the Site. Spaces will create your site boundary and display it in 2d and 3d.
The Site boundary works like other items, and you can edit it using existing sketch methods or the Vector-based editing.
With a site boundary defined, Spaces has calculated the site area for use in the Site coverage report and Floor space ratios report.