Ancient descriptions of the gods' dwellings on Olympus are pretty vague. There's a few references in Homer -- the Iliad refers several times to the fact that the gods have houses there, modelling the setting on a mortal city (the Roman poet Ovid does much the same, characterising Olympus as a version of the Roman senate); and the Odyssey mentions its climate, supposedly above the clouds, rain, and winds. But that's about all the detail we get.

Even at the best of times Robert Graves is entirely untrustworthy. Even in his supposedly 'non-fiction' works he cherrypicks details from all sorts of sources, frequently incompatible with one another, and he routinely makes up details out of thin air. In itself that isn't a bad thing, of course: the ancient sources do that too. So does Dan Simmons in his novels. But obviously they're different situations: Simmons' novels are presented as works of fiction, while Graves gives the impression that there's some kind of antiquity about the stories he's telling. There isn't: that's a flat-out deception.

In this case I presume what you've been reading is his short story 'The palace of Olympus' in Greek gods and heroes. It is entirely a work of fiction. Not a scrap of it comes from any ancient source. It's typical of Graves that he gives no hint of this in his introduction to the book.