Correct way to map a semi-detached house (and by extension terraced)

Can someone point me at information about the correct way to do this. I am mapping dwellings near where I live. They are mainly detached and I have encountered my first ‘semi’.

I have seen semi-detached houses mapped elsewhere (South of Reading) but it is not obvious to me how it is done.

I’m not sure exactly what yo are asking, but anyway…
Where possible (if aerial imagery is available), then buildings/houses should be mapped as areas (closed ways). And tagged as something like building=yes (or building=house), plus addr:housenumber, addr:housename, addr:street etc.

For semi detached/terraced houses, they should be mapped as adjacent buildings. ie areas with the middle nodes shared between them. How do do this will depend on what editor you are using. There’s a few different ways of doing this in JOSM - ie you can map one house area at a time (just make sure they are joined to each other, ie using the same nodes), or you could map the whole outline of the building, then split it into sections (make sure each section is closed way).

Note the Orthogonalise tool can be very useful for making the areas square, and the ‘Distribute nodes’ tool can be used for splitting ways in the middle.
You could also try the JOSM Terracer plugin.
It will automatically split a terrace outline into individual houses, and tag the house numbers etc.

Yes this is what I am trying to do for semi-detached. I think that I am doing it OK for detached houses with the correct tags. See:

I am using JOSM and the issue is how to get the two halves of the semi to share two nodes. I would like to draw the outline and then split it. How do I do that? Effectively how to a take a single rectangular closed way and split it into two closed ways sharing two nodes.

Thanks. I will look at the terracer but my problem now is just semis.