Architecturally speaking, sand has at least two potential problems as the foundation of a house. First, it can liquefy under certain conditions. Second, sand becomes unstable when it is shaken, such as during an earthquake, wind storm, or heavy flood. That's not to say that it's impossible to build a house on sand. With some clever planning and implementation you can certainly use sand as the foundation. Heck, people have built houses on marshes, swamps, stagnant water (see Venice, Italy), muddy mountain slopes (see Malibu, California), rocky cliffs, and more. Most houses, of course, are built on flat rocky soil. The reason for this is simple: Building a house on flat rock is safer, more reliable, and more cost effective than building on any of the aforementioned foundations.