2 semi-detached houses, one of which is a semi-detached house with a granny flat

Thank you for your info. But generally it would not be impossible to build like this if the development plan allowed it. First, a suitable plot of land must be found. I think it should have about 1000 sqm for all of us, so that everyone can also have a small garden and garages on it.

Thanks for your info. But in general, it wouldn't be impossible to build like that if the development plan allowed it. First, a suitable plot of land has to be found. I think it should be about 1000 sqm for all of us, so that everyone can also have a small garden and garages on it.

If I were you, I wouldn't fixate too much on that, unless you happen to have building land in abundance. I've been told that plots of land don't grow back ;)
In new development areas, it is often regulated that single-family house plots may be built with one residential unit and/or a single-family house with a granny flat (2 residential units).
Designated semi-detached house plots are often only allowed to be built with these 2 residential units, i.e. 2 semi-detached halves.
It's different with building gaps.
With some luck, you can get a plot where quite a bit is permitted. In the current case, a 4-family house was just discussed here a few weeks ago... three residential units would also be possible there.

whether an ancillary apartment can be built in one semi-detached house and not in the other

The number of residential units may be limited in the development plan; the building type "semi-detached house" itself does not force the total to only a "double" of residential units, nor does it require any equality of the two halves (except occasionally that the development plan prescribes, for example, equal "profiles" / roof pitches / eaves heights or the like).

... the building type "duplex" itself neither forces the total to be only a "double" of residential units, nor does it require any equality of the two halves (except occasionally when the development plan mandates, for example, the same "profiles" / roof pitches / eave heights or similar).

.... but only to the attached construction of both units ;)