There are many limitations to building underground including the quality of the soil, the water table, existing infrastructure and the requirements to making the structure habitable like fresh air and natural light. This is only a short list off my head of some of the items that you will need to figure out, they apply to most buildings, but in an underground structure some of them become more critical than others. if you live near a big city visit any underground garage and you will see some of these items that add complexity (and cost) to these structures.
