Nothing can ruin a great model building faster than seeing air underneath it. Most of our model buildings don't necessarily start with the excavation of a foundation. Some kits come with a molded plastic base which attempts to resemble a transition between foundation and ground. Most buildings are simply designed to sit on a level platform. There are some steps you can take to make these building foundations more suitable.
Of course unlike our train platforms, Mother Nature seldom yields a perfectly level and even building site. In fact, a model building nestled into a slope is seldom-modeled but adds a lot of character and realism.
You can sink your building into the ground by lowering the building, raising the ground, or a combination of both. To see how easy it can be, follow along with a typical installation on the Rio Grande Project Layout.