How near can extension be to old window

I am hoping to extend my double brick double storey house out to the side as the extension goes back I would be partially blocking the light into my own bathroom. This is not unusual as there is an old shed in exactly the same spot now. The problem is the shed was built years and years ago probably without permission. I hope to put an extension in the same place but do all the work legitimately. The window is almost immpossible to move does anyone know by how much in height and how far away I must be from the window to keep council happy. Is there a light meter or calculaton I need to do?
thanks in advance anyone who can help point me in the right direction.


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