Yes I got that he or his builder/architect drew plans that may or may not have contravened PD rules, (The chances are they didn't) he wouldn't have known this until he tried getting LDC, however, in certain circumstances as indicated in my above post, it may not be practical or possible so if an appeal is made on certain grounds, and accepted by the council, he can get is LDC as is without having to part demolish and rebuild 20cm set back, though i know Newham Council are very strict, but if you can put your reasons forward and say it was not possible to have that set back and TBH, it is not really needed on a side extension as such, so if you put your argument on the bases of structural or other circumstances, they may eventually be persuaded to accept as is. It is worth a try since no one really knows why they require this set back of 20cm on dormer being build on side extensions, on main part yes, is to do with rain water deflection, but where the roof slops to side and the rear most wall does not even come anywhere close to roof eves. Eves are on a side wall on a rear extension, and not on the rear wall of the rear extension. It may be hard to comprehend this in the head, so i will try to draw a picture later on to explain, and many council planners themselves don't know the rules or interpret them wrongly.
(technically speaking Eves of a roof is at the lowest part of the roof, where rain water gutter is attached to facia board, therefore on a rear side extension, the slop of the roof either runs left to right or right to left, and not from front to back or back to front, so the rule says that any wall of the Dormer should be set back 20cm from the eves, his rear extension does not have eves at the rear wall, but at the side walls, so as long as his dormer's side wall has an offset of 20cm, he does not have to have a 20cm offset from his rear most wall, it all about how you interpret rules and one can argue on those bases and appeal)