Do I need planning permission to rebuild a retaining wall?

I have a sloping plot of land with a stone retaining wall, and am currently awaiting planning consent for a single dwelling. My question is: can I demolish and rebuild said retaining wall and extend it a little before planning permission is granted? The entirety of the wall is within my land and about 15m from any boundary.

If the wall is no more than 2m tall then it might potentially enjoy permitted development rights to rebuild and extend. However, because it is a retaining wall, the council might feel that the works would represent an engineering operation. If so, the project would not fall under permitted development and you would have to make a planning application.

It would probably have made most sense to include any changes to the wall within the dwelling planning application. If it is too late to do that, then you can discuss the works with the application case officer; this should reveal whether the council thinks permission is needed or not.