Single story Side return extension and the 45 degree rule

We are thinking of having a single story side return extension built but I have only recently heard about the 45 degree rule. I think the extension would be PD as we only want to go to the end of our existing which was PD (3m) so the whole build would be 8m x 1.4m. Does the 45 degree rule apply? The build will go up to about 1m from the boundary wall which is already nearly 6ft high so I don’t think our neighbours would lose much more light. Thanks Steve