The third preference of both the customers are quite special ones, which cannot be incorporated into our model with mere vector analysis tools. One of the requirements is that solar panels can be used on the house efficiently. Of course, there are numerous factors to consider, like the exposure of the roof. However, the simplest feature such a house should have is the aspect of the land it is built on. If we are in the northern hemisphere, the terrain with northern aspect is mostly in shadow. In the southern hemisphere, the land with southern aspect is more shadowy. In reality, there are a lot more factors contributing to the solar energy potential (like latitude, climate, cast shadow, and so on), but for the sake of simplicity, let's just assume for now that the only factor is the aspect of the surface.
For solving surface-related problems, GIS has its most characteristic data type--DEM (Digital Elevation Model). DEMs are representations of a planet's surface...