Planning permission to reduce size?

If you wish to make a window or external door smaller, do you require planning permission? After all the window or door is already there and I could understand if you wanted to increase the size, but reducing the size.

I believe that any changes to a window size, is notifiable. Also, the window must be equivalent to at least 10% of the floor area in a habitable room.

If you wish to make a window or external door smaller, do you require planning permission? After all the window or door is already there and I could understand if you wanted to increase the size, but reducing the size.

Read the means of escape section of this in regards to Windows=https://www.planningportal.co.uk/info/200130/common_projects/14/doors_and_windows/2

Read the means of escape section of this in regards to Windows=https://www.planningportal.co.uk/info/200130/common_projects/14/doors_and_windows/2

Thanks, very helpful, especially the case study.