I don't have any idea of if there are any 'rules' of proportion that one should follow, but visually I would balance with the other window and go with that height..... can never have enough natural light! Your drapery could go full length to the floor.

Might have to rethink drape length as i see radiaters under the windows - maybe vertical radiators ?

Thank you both for taking time to read & respond to my question.

Kazzh good thought about the curtains but the photo was from previous owners decor, furniture etc.

Siobhan good point, my thought was the same but I can’t decide if it’s necessary next to the table? There are radiators but luckily the one by the dining table is low, same height as the radiator under the other window, (almost as if someone previously had the same idea). However, we have checked & planning permission is no longer required & we have spoken to the neighbours.

A complete quandary really I just wondered if there‘s a design rule that says window sill height should be the same?

There is no design rule for window sill height, its all down to preference some people like floor to ceiling in their bedroom but others dont, personally i wouldnt have floor to ceiling in my bed room on the off chance someone looks in but its all choice. If you want a lower window sill i would go for it, as id like to look out the window from the dinner table whatever the view. if you lower it once the window is out u have the option to have the same size and frame as the other one.

I had this issue when we were renovating our current home and we aligned them on the advice of our architect and it does look better even though these windows are in opposite sides of the room.