Beginner FreeCAD question on axes when sketching
This answer assumes you're working in the Part Design workbench.
So you want to adjust the starting height of a pad or pocket. There are two ways I can think of to do this easily.

Adjust the Z attachment value of the sketch to lower the sketch below the surface.
Use the &quot;two dimensions&quot; option in the pad/pocket operation and set the second value negative.

By the way, it is highly suggested not to attach a sketch to a surface. The current development version (0.19) and previous versions have an issue (&quot;topographical naming&quot;) where editing a previous feature can change the internal name of a face causing the attachment to break. There is a solution under development but it doesn't loom like it will get in earlier than 0.20. You should instead simply adjust the Z attachment value to match the face you want.
Also, there is a very active (and very helpful) forum here. I'd suggest asking further FreeCAD questions there.