Changing agent height

Back in Edit mode, you can press 'H' cycle through a sequence of different agent heights.
The current height is displayed at the bottom of the window.

Change the height to the smallest value, and switch back to Test mode.
With the new height value, the 3D content processing will leave valid ground underneath the areas with lower clearance.

In this scene the shortest agent fits underneath the table.

Try moving the shortest agent under the table or underneath the fence sections, elsewhere in the scene.
Try generating for the tallest agent, this agent shouldn't fit underneath the smallest of the three arches, and will choose paths around that arch.