Building a home, or portions of a home, underground as a subterranean space provides a variety of benefits both environmentally and for practical living opportunities.  Rock, concrete, and soil are all great elements for sound attenuation and energy efficiency.  In a hot dry climate, these elements keep the interior space cooler, in a cold wet environment they offer dense protection and reduce heating requirements. 

Additionally, Zoning and codes governing square footage, setbacks and density are often waived if the square footage is fully subterranean; allowing for more livable area than would be possible if the entire structure was built above ground.  When a building site is on a significant slope, subterranean space is not only practical but can be desirable for access.  Building the garage subterranean on a steep site often reduces the driveway grade and generally offers opportunities to house a larger number of cars than would be practical on grade.