For some reason last night i was thinking about bunkers..don't really know why but that led me to think do you need planning permission for something that is underground? as it isn't exactly blocking the view is it..
Don't know about planning permission, but we'd love a bunker in our back garden. The thought of getting away from everyone & to sit reading by candle light! How cosy.
Where I live you would not need planning permission provided the structure was wholly underground - think about your entrances - but would require building regulations. My friend has an underground room in his garden under the decking where we play snooker which he did not require premission for - we checked.