Replace fixed obscured windows to clear and/or opening window - Permitted dev?

[hi4]

Can a neighbor replace fixed obscured windows to clear and/or opening window under permitted development or without planning permission?
We have recently bought a detached bungalow and our neighbors (detached bungalow) had two fixed obscured windows (their living room) on the side and they are planning to replace it with clear opening windows on the side facing which overlooks kitchen and a bedroom. We came to know this while they were chatting to us and mentioned between them. We thought it is a strange way of asking us, perhaps trying to gauge our reaction!? We are not happy with the plans. The space between two bungalow is 2.8 meter approx ( 2 meter ours and 0.8 theirs).
I was trying to find similar question on the net in relation to ground floor window but found none. All I found was information related to upper floor and new extension should have 11 meter gap in order to have side windows. None was related to replace fixed non opening windows to clear and/or opening window on the ground floor.
Our questions:-
Can a neighbor insert clear and/ or opening window under permitted development on the ground floor?
We would like to understand our options if replaced it with clear opening window without planning?
Does anyone had similar experience and/or information that can help to understand the options.

Advance thanks for your inputs.

[JK0]

Tell neighbour you will plant a dense holly hedge if he does.

[Interlaken]

Can a neighbour replace fixed obscured windows to clear and/or opening window under permitted development or without planning permission?

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When this property first got planning consent was any mention made in the conditions of the windows having to have obscured glass? Time to investigate at your local planning office.

If not it is entirely up to the householder as to what to do. I assume this is not 'alterations to front elevation'?

A few years ago some LA's were trying to get householders to apply for consent for DG windows - a method of revenue creation I suspect but the practice seems to have fallen by the wayside.

I like the idea of the giant holly hedge