I expect that the game will only allow cars to park on what it recognizes as the ground level. That is, there must be ground underneath it. Have a look at
this tutorial on building an inground garage, and it might give you some ideas.
What you probably want to do is build up the lot a floor high, and then dig out a hole like in the video back down to normal ground level. You can then build the first floor, minus the garage door and ramp of course, and then build the foundation around it as walls before flatting out the land to expose the first floor. Essentially, build the first floor in a hole, then build the second floor, and then expose it.
Or perhaps someone has done this, but that's my best guess.
