TOPIC:

If you have a plan from OS then it would most likely be in .dwg format, which would require Visual Building Pro to import. Alternatively all versions could import such a plan as a bitmap.
The scale of such a site plan would normally be such that it would be a different view and scale (typically 1:200) to that upon which your 2D building project is placed (typically 1:50). (depending of course upon the size of your site).

It is not essential that your site plan / block plan appears in the same project as you building construction plan, and it is often easier to create such plans as a separate project and import them into your main project as a bitmap.

Remember that Visual Build Pro and above will allow you to layout your views allowing multiple views per sheet, whereas with Visual Building Basic and Lite, you are restricted to 1 view per sheet.

The 2D Graphic tools are used in this video (however not creating a site plan, but the principle is the same)

I could import or draw my own.
My site is flat so no contours.
I intend to make a planning application.

Creating a site plan depends upon several factors:
Are you intending to import an OS plan or draw your own?
Do you want to show contours / slopes?
What is intended purpose, Planning application or marketing?

The features list for Visual Builder markets the Site Plan capability (a basic for any building planning really) and yet searching the help topics and the user manual for "Site Plans" produces no result. How do I get started please?