How to handle residential building with vertical split?

I would like to map a building. It has two floors. The ground floor (level=0) is 35, Maes-Henffordd, Cardigan and the first floor (level=1) is 36, Maes-Henffordd, Cardigan. Like flats except they have their own house numbers. And there are three more, just like that, and they’re all joined together in a terrace: 4 house numbers on the ground, 4 up on the first floor.

So what’s the best way to handle it? I couldn’t find anything on tower blocks that looked relevant and nothing at all on tenements. My first thought is two superimposed buildings with level=* to distinguish one from the other. The only other thing that looked relevant is to use a range, but that is often used (in UK postal addresses, if not on OSM) to indicate an organization that occupies what was once two (or more) conjoined buildings that have had internal walls removed. Is there a better way or are those as good as it gets?

building:part, rather than building

I also feel that vertically stacked building:part areas would be the best approach to represent this situation. While it’s more commonly used in the context of 3d rendering for parts of the buildings that are visibly different (e.g. one part is painted blue, the other is painted red), I don’t see an issue with using it for a non-physical property in this case.

Yeah, it makes sense. Problem is, it’s a pain in the nether regions to edit. I tried it. I may be missing something obvious (I probably am) but I have two superimposed objects (with no non-overlapping lines) and I can’t persuade iD to let me select the bottom one to make changes to it. Which I’d like to do, because I inadvertently left in building=house for the lower one. Which means to fix the problem with the lower one I’ll have to delete the upper one, change the lower one, then add the upper one again (unless somebody tells me the feature in iD that will let me choose between them).

Which leads to something else. If I use the query tool in the map, it shows me there’s a house (because I accidentally left in building=house) and a way (because the upper one is only a building:part=house, which is classed as a way). Yeah, it’s a cosmetic detail and mappers can learn to live with it, but it’s another problem.

So this may be the right thing to do, but it’s not without problems. Although this is the first time I’ve encountered this arrangement in Cardigan, I know of a very large housing estate elsewhere in the UK composed of nothing but dwellings arranged like this (I just checked and although the roads are there, none of the buildings have been mapped).