What do you call a circular paved protuberance added to a paved street?

A cul-de-sac (or no-through-road) is — at least in British usage — normally a separate road/street in its own right, with its own road name.

In the village where I live, we have a modern estate where two groups of houses are accessed by a 'shared driveway' as shown in the Google satellite picture below (see top right & bottom left).

These are not circular in shape, but they do meet your description of maximising the use of land by several properties while using little immediate road-side space. Here, they are not 'extensions' of the main road but akin to shared 'driveways' because you do have to cross the pavement (AmE sidewalk) to access the driveway, just as you would for a single house. I do not know of a specific name for this type of arrangment, but if there is one, it could well differ from country to country.

I'd say turning area was the most common term, sometimes turning circle, but that could be confused with the space required to turn a particular vehicle around. Incidentally they're now normally Y- or T-shaped in new build, because that makes it easier to turn large vehicles (rubbish lorries, fire engines, etc.)

Continuing the tradition of pretentious diction you could borrow the French term fond de raquette (French for "bottom of racket"), or you might want to combine the two languages and label it as a turnaround cul de sac or a turnaround impasse.

In London these configurations are quite common and usually referred to as grove and close. There is an East End street called Frying Pan Alley, although in this particular instance the name is believed to historically reflect the presence of braziers and ironmongers.