What you are really saying is that neighbours should have a veto - basically an arbitrary property right - over your ability to sell your property to the highest bidder.  
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Have they purchased that property right?  No.
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Is there any basis in law for such a right? No.
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The idea is NIMBYism run wild.  The same basic arguments were used two generations ago to try to enforce "covenants" not to sell to black people or Jews.
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Have we learned nothing from that experience?
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If you don't like your new neighbours, or you don't like that they have received planning permission to adapt the property to a higher valued use, you are free to sell your property to the highest bidder, accordingly.
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What your neighbour has done, most likely, is to make your property more valuable.
If the neighbour were imposing a negative externality on you, you might have grounds for complaint.  But you're gaining a windfall - and still complaining!