Extruding buildings to a random height from an assigned topography.

Hello everybody, 

What I'm trying to do is creating an architectural site model using the Random button.

I followed this tutorial from the Digital Toolbox.

It's all fine up until the surface I want to extrude the buildings from is non-planar. What I obtain is buildings with roofs parallel to my topography.

Is there a way to extrude a building with a flat roof from a non-planar surface?

Or how can I obtain the same result still using the random button?

The other thing I tried is baking the curves I projected onto the topography into Rhino and then Lofting them up with the flat curves I moved randomly along the z unit. It does work but the problem is the process is too slow (After having randomly pulled everything up I'd have to project, bake and loft for every single curve) and the laptop easily crashes when handling slightly bigger amounts of data.