I wonder if part of the problem may be that the Dutch/Afrikaans word stoep is not a true cognate with the English stoop (except perhaps in South African English?). It sounds as if stoep might just mean porch in a more general sense.
Marking the existing entry South African might or might not be useful, but even so, it would only address part of the problem; you would still need other options to translate 'stoop' E>D.
I checked the monolingual dictionary to see if definitions would help differentiate among the various choices in English, but Webster's at least is very vague, not really reflecting usage as I know it in practice.
To me a stoop is a subset of porch, the kind of porch that is small, not much wider than the door, consisting mostly of steps. It wouldn't typically have any railing or wall on the front side, only steps for the full width, and it wouldn't be large enough for any chairs, only for people. I don't picture most stoops as having a roof over them at all, though I suppose one could. I think of a stoop in front of a townhouse such as a brownstone, or in front of a relatively small, compact house.
A veranda in English is perhaps a different subset of porch, or a word used for porch more in some countries or regions than others. I picture a veranda as extending across the full width of a fairly large country house, perhaps even going around two or more sides of the house, and having furniture on it and being used as if it were another room of the house, just an open-air room.
But Veranda in German may not cover the same range of meaning; it too may just mean generally 'porch.' And other English speakers may have a different mental image of what constitutes a stoop or a veranda as opposed to a porch. Perhaps others will comment.
Sage, to answer your question, the villa in the Geiger novel is evidently in a well-to-do suburb of Vienna; the late grandfather who owned it was a government minister in the postwar period, so I assume it's fairly substantial, though not necessarily a mansion. I think the modern grandson who inherits it can sit casually out front on the steps and have a roll and coffee for breakfast without needing a chair or a table, but who knows, it might actually be a bigger porch and he's just sitting on the porch steps. I'm not sure if there is a front yard (= garden BE) or only a back yard, but in front there is at least enough space for a driveway and sort of graveled parking area, IIRC.
Maybe Austrians will comment on what they picture a Vortreppe as consisting of, or there might be a fuller description of it in a dictionary.