This is pretty simple. This is from the help file:
To use these attributes with the CGA grammar you need to declare CGA attributes with matching names. A simple CGA rule file could look like this:
attr height = 10 Lot --> extrude(height)
After assigning this rule file to the shapes, the CGA attribute height appears in the CGA Attribute Mapping tab of the Inspector. Note that the Source field is set to Object, which denotes that the CGA attribute height is controlled by the object attribute of the lot.
So in your case this is all you would need to do
attr Your_Shapefile_Field_Name_For_Floors = 10 #value can be anything as it will map to the data
attr avgFloorHeight = 3 #setting this as an attribute will give you the ability to change in each building if necessary
@StartRule
Lot --> extrude(Your_Shapefile_Field_Name_For_Floors * avgFloorHeight) #again you could hardcode "3" here
