It partially depends on where the car sits at the moment. They do seem to vary in ride height out of the factory!
I fitted a 50mm lift to ours and needed to cut the inner wing right down to the 'chassis' member. It needs a bead of weld to replace the spot welds IMHO. I reckon ours would have needed some cutting even with a 40mm lift. The other problem, at least on the TD4, is the wiring harness runs along the top of the LH box member. This is also fouled by the track rod so needs relocating.
You should also check the camber after lifting to make sure it is within tolerance. We had to adjust the front with offset pins. The rear was fine. I have the factory data sheet somewhere if you need it. Obviously, the tracking will also need resetting.
HTH.
