In everyday usage ‘free plan’ and ‘open plan’ are often used interchangeably. They are, however, very distinct. A free plan comes into play when the role of structure is separated from that of enclosure of spaces. Generally, it arises through the use of columns that allow the partitioning of space to follow a logic other than that of the structural grid. In the best demonstrations of free plan (see Le Corbusier) the columns play a spatial as well as structural role, defining sub-zones, passage zones, or used sculpturally to create a greater sense of depth in space. An open span is a plan which is left largely unplanned, as in typical office towers or single space homes. In the first (free plan) there are always rooms, even if they are not entirely closed off, while in the second (open plan) there is only the perimeter.
A raumplan is a planning method based on discreet rooms and a dynamic section. This method places great emphasis on the scale of individual rooms and often requires steps into each room or cluster of rooms. The method largely belongs to the architect Adolf Loos and requires a high level of structural awareness and ability to model spaces.
Source: Le Corbusier & Adolf Loos. See Raumplan versus Plan Libre by Max Risselada.