Question continued: For example, where do you position the planar surface, and how do you calculate accurate space volumes? Is this up to the authoring tool, or does gbXML provide guidance?
Answer: gbXML files as you’re alluding do not take wall thickness into account. All authoring tools are instructed to take the wall’s centerline as the planar surface location in gbXML. The volume then typically is calculated only assuming half the wall’s thickness. The floor area also only takes half the wall thickness. For most buildings where the wall is not thick relative to the floor area or volume. However, for those buildings where the thickness could cause a significant error in the area and volume calculation … then consider another workflow where the full thickness is taken into account. I would suggest drawing a simple shape and export the gbXML file. Open it up and look at what it is doing. Also, consider submitting a Test case for Thick Constructions.