Floor Height specifies the ground level of the floor

[LaNaStL]

Supposedly one floor of a plan can be designed lower than another floor according to Help as follow:

Floor Height specifies the ground level of the floor. This is useful in designing split-level homes. The default is 0, but you can specify a negative number to make the floor lower in one room than another. When you set this value, the Ceiling Height value automatically updates to keep the ceiling level across the plan.

However, trying to show living area rooms 24" higher than the garage cannot be accomplished - either by increasing living area rooms (+24) or decreasing (-24) the garage.

So...how is this done?