Final answer:
The area of the stairs should generally be added to the ground floor's RMS area as they provide access to the second level from the ground floor. This is typically the practice followed in multi-story buildings.
Step-by-step explanation:
To determine which level of a property the area of the stairs should be added to when calculating Room Mean Square (RMS) area, it's necessary to understand the layout and design of a house.
Normally, in a multi-story building, stairs that connect one level to another are included in the RMS area of the lower level, because this space is necessary to access the upper levels. In this scenario, the stairs that lead to the second floor would typically be counted in the ground floor's RMS area, as they provide access to the second level from the ground floor.