In summary, yes it is trespass, but if it has been up for a length of time (12 to 20 years...it is a bit vague) then the you may end up formally losing your "airspace".
Furthermore, the courts tend to view this as a trivial trespass, so are unlikely to do anything anyway it seems.
So as I mentioned, it is imperative tackle such problems before any extension is built, as the options after the fact get more and more difficult as time goes by.
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