Requirements

A parking lot consists of many spaces on many floors.
A spot can be identified uniquely.
A space may not be rented to more than one car at a given time.
A driver may rent a space for the size of their car.
A driver may rent a space for a calculated fee based on how long the space is rented for.
A driver needs to be told if a spot is available for a given size vehicle.

Assumptions:

Let's assume the parking lot has 3 levels.
Each level has 3 spaces.
The parking lot has 3 spaces for small cars, medium cars, and large cars, respectively.
A space is rented for $10/hr regardless of size.
No 2 cars may rent the same parking lot at the same time.
Let's assume drivers leave their space after their allotted duration is up.
One driver can rent a space at a time.