Well, you say that the stamp tool is not an ideal solution - however all your description (what you want to achieve) is about how the stamp tool works and what it does.
Is there really 4 metres difference in that building? As I can imagine, such a building wouldn't be placed on such a terrain with all the terrain built up / dug into around but the building itself would be somewhat on a sloping foundation.
Note that you can always stamp a slanting face or even a collection of (joined) faces that are not coplanar - the only thing you cannot stamp is a face perpendicular to the terrain (i.e. basically vertical faces). See what I mean in attached:
Stamp.jpg