Make sure that meshes that are placed on the ground always have a foundation.

Some of our meshes are meant to be placed on the ground. The primary example would be the structures, like houses and etc. These should all have a foundation, which stretches at least two meters downwards from their base (i.e. 0 on the vertical axis).
The effect currently is that a lot of the structures look good when placed on a totally even surface, but looks strange when there's unevenness (which there usually is).

This also applies to all of our trees, with the difference being that these instead always should have roots.