Scale whole object in normal orientation As the title says, I want to scale an object perpendicular to the normal of a face.

In edit mode it works fine when I set the transform orientation to normal mode.
But in object mode this isn't an option I believe, local orientation doesn't work eihter, because the rotation of the object. 

I can eyeball the scale in edit mode or rotate it in object mode, but it needs to be precise for architecture.

Thanks in advance!

Normal orientation


Wrong scaling orientation in object mode (local orientation)


Maybe there is an method to apply the rotation of an object to a normal or something?
In Edit Mode, create  a Custom Orientation from the face. (That's the '+' in the orientation dropdown at the top of the view, in 2.8 .) Check 'Use After Creation'..Then back in Object Mode, use it, with SZZ

However, it's probably better to A select the whole mesh in Edit mode, and scale that, because non-uniform scaling of the object (giving the object a transform matrix including a non-uniform scale) often interacts in undesirable ways with modifiers and constraints. Either that, or when you're done scaling in Object mode, CtrlA, bake the scale into the mesh by applying it.