It really depends on the complexity of the ask. Are you trying to overlay building plans on a map, or just map points on one? If the former, you're now talking about needing custom polygons in addition to the actual mapping service. Building custom maps is fine, but you'll usually need a web mapping service (WMS) server to host/serve these if you want them to behave like normal maps which support searching/panning/scrolling.
If you just want to create a static version, you should be able to overlay regular polygons on a map. Perhaps a bit more clarification would be helpful?
If you just want to look at a few things which may help you in this regard, you might try some GiS software, or a combination of things like Mapbox (which is natively integrated into Tableau), in conjunction with Interworks' Power Tools (which is free).
I'm looking for the former. Actually I'd like the map completely blank except the building plans. Can I do that in MapBox?
I'm not sure what you consider "completely blank". If this is truly a white/blank background, you shouldn't really need maps at all, as I assume you don't have lat/long coords for every corner of the building. I would probably just overlay all the floor plans on an image with them spaced in a rough approximation of the distance between the buildings. I would then trace over them with the Interworks tool I mentioned.