Yes, it does depend on what the sub-base is under what's there now.
For a car, it really does need a good solid hardcore sub-base.
Assuming that there is hardcore there, I would excavate what's there to a depth of 100mm.
Then pour 100mm thick concrete. When the concrete has gone a bit stiff, then add the pebbles. Use a piece of 4x2 (100 x 50) timber to lightly tamp the pebbles down into the concrete to give them a good seat.
Concrete mix: 4:2:1 That's 4x chippings, 2 x sand and 1 x cement.
But, I repeat, an excavation hole is essential to make sure there is a good hardcore base.
If not, then excavate to a depth of 250mm. Ram 150mm of hardcore, then the concrete and pebbles.