How to define a suporting wall for a plot on a hill

[AlbertVonMoosseedorf]

I am new with SweetHome3D and I found two situations that I don't know how to solve it. Both have the same problem: When defining a wall, i can set the hight at the startpoint and at the endpoint. This will result in a wall that elevates from the ground to top like a ramp. But in my case, the top is horizontal and i need the wall growing downwards.

Any ideas ?

[Puybaret]

Please read the tip How to create a hole at the bottom of a wall.