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?
look into the sandbox in tool menu
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.