make non planar surface with curve

Hello,

I come to you because I have a worry, I am a student of architecture and I am in the process of modeling the study ground for my project.
I created the topography via ELK + geotiff
And outlines of buildings via MEERKAT + shapefile
To extrude the concrete from the surface of the ground, I projected the layout of the buildings on the topography surface. But then, it is impossible to create surfaces from these curves to extrude them. If I extrude from the curves, my buildings have no roof.

An idea ?

Thank you in advance :)

 In Green : The result of Patch

In green : The outlines of the buildings on the topo

P.S : sorry for my bad english ...