13m rule and 21m metre rules are mentioned elsewhere
Usually stated in local authority Local Planning Guidance Note (LPG)
Here is an example grabbed from Wrexham Council LPG
“Where two habitable rooms face one another such that direct overlooking is likely to occur, the windows shall be a minimum of 22 metres apart. Where a window faces a wall which exceeds the height of the top of that window, they must be a minimum of 13 metres apart. This standard applies on flat ground.”
