[GaryVoshol]

As low as you want. Just remember that there are bumps for the lane divider supports that are about 1/2" tall.

Bumpers must be within a 6" zone, between 2.5 to 8.5 inches from the floor. Since a bumper is 5" wide, you have the option of where that extra inch goes. In other words, the bottom of the bumper could be from 2.5 to 3.5 inches from the floor.

You can have parts of your robot that are lower than the bumper, but they cannot be sticking out underneath the bumper. The bumper forms the outer surface. Obviously a manipulator of some sort is exempt from staying within the bumper area. You must take care that you do not damage or wedge another robot with your extension.