Building pad below existing topography

Hi all!
I have a topography which I’ve imported from CADMapper. So I wish to build below the topography in areas. In Revit you can make a building pad on the toposurface which will allow you to put a slab below the existing ground. How can I do this on Sketchup?
Kind Regards,

look into the sandbox in tool menu
http://help.sketchup.com/en/article/3000135

There are some decent videos on YouTube about the Sandbox tools. It sounds like you need the Stamp tool.

The cleanest way is to intersect a footprint of the building with the terrain surface.
Then simply delete the unwanted area of the surface.
See this model:
Cutting Terrain Holes.skp (616.7 KB)

The Drape tool can be used in much the same fashion.
In certain situations the Stamp tool tends to create a mass of unnecessary geometry.