Hi Frank,
I'm afraid you probably haven't got a lot of options in a situation like this.
According to the Council's interpretation of the term "eaves", your dormer wouldn't accord with the requirement for a 20cm set-back. Because this this requirement is ambiguous (e.g. the government hasn't provided any definition or guidance for the term "eaves"), the Council was fully entitled to take such an interpretation, even if there are (or were) examples of other Councils that take (or took) the opposite interpretation, and even before there were any appeal decisions on the issue.
The fact that there is now an appeal decision that supports the Council's interpretation 1) strongly indicates that the Council didn't act unreasonably in taking such an interpretation and 2) weakly indicates that the particular interpretation that the Council took was the right one. I say "weakly" only because one single appeal decision on an issue is not particularly conclusive. Indeed, if (say) 5 appeal decisions are issued next month that support the opposite interpretation, or if (say) the government releases guidance next month that supports the opposite interpretation, then you could show these to the Council, and ask them to reconsider their decision. However, even if this happened and the Council changed their interpretation, it wouldn't mean that they were unreasonable to have taken the interpretation that they did at the time when they did so.
Where a Council hasn't acted unreasonably, but you disagree with their interpretation, your only real option is to submit an appeal to the Planning Inspectorate (other options such as the Ombudsman, judicial review, or even a costs application during an appeal only really apply where the Council has acted unreasonably). There is always a possibility that the Inspector who determines your appeal decides that the interpretation of "eaves" taken by the Council in your case, and by the Inspector in 136 Wood Lane, is incorrect. However, as previously said, if there are also other more clear-cut reasons why your dormer is not permitted development, such as because it exceeds the volume limit, then there would be little point in such an appeal.
The other thing you might want to do is to have a look at the "Appeal Decision Summaries" document on my website (
www.planningjungle.com), to identify all of the cases where a dormer was allowed, and to look through all of the associated documents for these cases (which are also available on the above website). I'm not aware of any other appeal decisions (other than 136 Wood Lane) where the Inspector commented on whether, for the purposes of the 20cm set-back, the eaves should be taken as the outer edge of the overhanging roof tiles, or the point where the roof tiles meet the main wall. However, I certainly don't spot everything in these appeal decisions, and I may well have missed such a conclusion, particular if there was a case where the drawings showed a 20cm set-back from the outer edge of the roof tiles and the Inspector allowed the scheme without directly mentioning this issue in his report (which would still imply support for such an interpretation). I'm afraid that looking through the appeal decisions and associated documents to see if there is any such further information would be quite time-consuming, which is why I'm afraid I wouldn't be able to do it myself.
Anyway, I hope the above helps,
Steve