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I get what you're saying but still don't get the £15k figure - where has that come from? Has a solicitor advised him of this? It seems unlikely given how woefully inadequate most local authority planning/enforcement teams are.
If your bro thinks the neighbour has got off lightly he can raise that with the council's enforcement team, but every planning application is unique so I can't see why they'd pay anything out.
Also, if he hasn't already, his best bet is to appeal their decisions. If he hasn't already done that there's definitely no point involving a solicitor.
His neighbour hasn't built the same extension. His neighbour didn't accommodate the changes that the council wanted my brother to make. Hence the figure of fifteen grand.