Generally a window within a certain distance of a boundary which overlooks neighbouring land is not permitted.
Have you checked with the council's planning department to see if the neighbour obtained planning permission. You state it is a large extension and this may have required planning. The council may not have informed you for 2 reasons (1) it is permitted development or (2) they do no know about it.
You should speak to the planning department and they will come and inspect.
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