The work plane does not have to be either the start or the end of the extrusion depth. It is only used to sketch on and set the extrusion direction to be perpendicular to the plane.
You can set the depth of the geometry before sketching it. In sketch mode, enter a value in the Depth text box on the Options Bar. This value changes the end point of the extrusion. Depths can be negative values.
You can modify the depth of the extrusion after creating it by selecting it and on the Properties palette, specify values for the start and end points. You can also resize the extrusion in a 3D view by selecting it and then dragging it to resize it.
If specified, Revit does not retain the end point value during creation of the extrusion. If you need to make multiple extrusions with the same end point, first sketch the extrusions, then select them, and then apply the end point.