Thanks for pointing out my errors Matt.
The erroneous notes have now been removed.

Jez, would you be kind enough to exemplify a terrace so that I can see how it is supposed to be done?
I keep running into a "building within building" validation error.

The problem I have (putting 3D aside for the moment) with two or more building:part = yes contained in a building = yes is that I no longer see the house numbers in OSM.org

I've just watched that. I hope everybody sticks to the Urbex ethos.
I've done a bit more 3D work on the site this evening.