Putting everything at two levels feels incorrect. I have tried mapping the main building normally, and then adding a Building Part with one level. But that does not look very correct either (67B and 67D).
Thank you CjMalone for this example, have thought for some time that the wiki pages could use more examples of what things should look like. I’ve subsequently held off from trying to map building parts as a result. Though @CocolinoFan I do also recommend taking a read of Simple 3D Buildings - OpenStreetMap Wiki , as despite lacking examples it is worded decently, and clarifies additional things such as relations.