Unfortunately Civil 3D sucks as a BIM product, so we are going to have to do a work around in order to get this to work. The first step is to create a hatch, or hatches, representing the floor of the structures we have to show. Once we have this we can then create blocks for each one, making sure to give them a adequate name. Next create a label for each size having the hatch as it’s component. Make sure to rotate the block so it points up. Now adjust the attachment point for the structure labels to the bottom of the structure.
If you live in the real world, and not some fantasy world of Autodesk’s developers, user interface designers and the like, then you will probably have requirements to place labels at the top, middle, and/or bottom of the structure in one plan set. I know crazy thing we plan producing people have been doing for decades is beyond the comprehension of people who don’t put together plans. For this label you will want to anchor to the bottom of the structure. You do this by going into Toolspace, Settings, and adjusting the attachment point for the structure. Once you add the labels make sure you go back and adjust it to the correct location on the structure for your other labels. You’ll be glad to know that changing the color of the user interface to dark is more important then making it easier to change the location of the structure labels.