If your basement has flooded in the past or you have not lived in the house long enough to know, a basement subfloor may be necessary. A basement subfloor is typically used to create a buffer between the carpet and concrete. For information on leading methods you can view a video featuring Bob Vila.  

In Johnson County, most newer homes built in the last 15 years have been built with the idea of having the basement finished at some point. This is why most new homes in Johnson County have a sump pump already installed in the basement. If this is the case you likely will not need a basement subfloor.