David, typically referred to as "berm" houses, but I believe that most cost manuals refer to them as "earth contact" houses. Basically, it is a basement with a roof and one side open... sometimes two sides open, and I have even seen them with three sides exposed. Regardless of whether your local market refers to them as "underground" houses, that is not what they are. Underground houses are characterized by a roof that is covered or below grade.
Usually there is some functional obsolescence when compared to above ground houses. That is because the floor plan only allows for exit out the open side. If they are constructed correctly, the open side should face south. Also, there can be big differences in construction and finish quality... good ones will have blue board insulation around the outside of the below grade walls. Every now and then you will see one with a second story that is entirely above grade... this is still somewhat different from a raised ranch because the kitchen and other living space is usually still in the below grade section.
Although ANSI says anything with any portion of the floor level is basement, there is an exception for these houses because the items necessary for living are all below grade by that definition. Therefore, you may call the finished living area GLA.
However, you should compare these to other earth contact houses and not to above grade houses. That can lead to problems, depending on the market. I took a little trip through an area of mid-Missouri this weekend and I believe three out of every four houses we saw in one area were berms. In a more typical market area you might be lucky to find one berm sale within a mile within the last year. Obviously, in such a situation, you have to expand the search area and time frame a bit.
