We did, without difficulty .
But it depends on may factors.. DS1 has learning disablilities (mild/moderate) too, attended special school etc so I suppose he was considered more needy .
Having said that, I typed up each question and wrote a clear picture of his daily life, what we do for him, and why, along with examples of what happens if he is not supported (eg at 18 I would expect him to be able to make a simple meal/ catch a train/ find his way to a new place..he cannot do any of these things ...and I explained why)
PIP isn't about the diagnosis but the actual care needs. He even has a part time job now, but that has been with support from MENCAP to get him started and amazing staff at the supermarket and I detailed how they have supported him.
I explained clearly how his autism impairs him in every aspect of daily life, and I repeated myself a lot.
I did include a few reports but once he left his special school , we have barely seen anyone since, so it was mostly down to detailing every bit of help he needed.
His was granted 'ongoing' (which is 10 years max now not lifetime) so it must have been sufficient. I think I bored them into submission!
Good luck, and just put in as much detail as you can!