Updated: Aug 3, 2019
In building design, the most primitive requirements in the building program is the areas allocated for each space. It is for this reason that understanding how Revit computes room area is absolutely critical.
In Revit, there are two basic Categories that address how areas are measured in the model, they are Rooms and Spaces. Rooms contain basic information such as area and can include volume if it is turned on; by default volume computation is turned off because it may adversely affect performance. Rooms are an architectural category and intended to help designers define the building program. Conversely, spaces are an engineering type category and are capable of containing a much larger dataset such as heating & cooling loads, airflows and information about lighting levels. In order for either of these categories to function properly in the model they must be enclosed by room bounding elements such as walls, floor, ceilings, etc.
By default, the following elements are room bounding:
Walls (curtain, standard, in-place, face-based)
Roofs (standard, in-place, face-based)
Floors (standard, in-place, face-based)
Ceilings (standard, in-place, face-based)
Columns (architectural, structural with material set to concrete)
Room separation lines
You can change whether an element such as a wall is room bounding or not by turning off the Room Bounding check box in the properties; for example, perhaps you didn't want the closet in a bedroom to be considered separately from the main room. As seen in the images below you could simply toggle the room bounding property off for the walls enclosing the closet.