I want to use the cutting tool add a new edge just above this doorway. I need the tool to cut at exactly the same height all the way around. I don't want to use Ctrl+R as the loop cut cuts other parts of the building too:
Try the Knife tool, set your view to an ortho side view hit "K" for knife than "Z"(cut through) and "C"(constrain) than draw your edge cut.
EDIT: Another option could be to select these edges and just right click and select subdivide and than slide that edge by double tapping "G" to move it into position. Creating edge loop cuts is usually easier when you don't have Tris though.
As an alternative to Chris's solution, I think you can achieve this pretty easily with CTRL+R (loop cut), hiding the faces you don't want to cut. Suppose I want to add a vertical loop cut to this cube:
But I don't want it to go the full length of the cube:
To avoid this, I can switch into Wireframe view, and select the bottom 4 vertices:
Hide them by tapping 'H' on my keyboard:
Then add my loop cut:
Finally, hitting Alt+H to unHide the hidden vertices, we see that the loop cut only cut the faces that were not hidden: