Room additions in Rapid City are considered to be a major renovation and construction project since it means making an additional structure that you will connect to your house from the outside or within. The cost will greatly depend on the size of the room that you want to be added and where you will place it. It will also include the materials to be used and the labor cost. For this project, it is highly advisable to hire a builder or contractor to help you assess your options. You can have your room additions build in two different ways; either by building out for by building up.