When I place a house on a lot it was not originally built on, I get either mountains they sit on or they sink into the ground.
This usually happens for lots that were build on a different lot than you are trying to place it.
This problem can be solved with a cheat. You will need to type in the cheat before you place the lot:
(value) Is a positive or negative number depending on whether the lot created a hole or a mountain.
After you placed the lot, check if the elevation is level before pressing ok. If not, cancel and change the value you put in. Positive numbers raise the lot and negative numbers lower the lot. You can even use decimal values for fine tuning.