Chertsey platform lengthening is a part of NR's CP5 Reading via Ascot line 10 car and power upgrade, and work on the ground for that whole project was shown as starting in April 2016. (Pending any Hendy review delays that may yet become apparent.)
Can't explain why it is included as an off-route part of the overall project. I can only assume that it was a late change, the 10 car suburban project always showed Chertsey covered by ASDO.