My neighbour reckons I need planning for my (elevated) garden room

Burying a prefabricated garden room 300mm or more underground is a bad idea. Putting aside the cost, water ingress and accessibility issue you'll have additional steps between the 12.m level and the 1.5m level. If you were walking from the 1.5m level down into the 1.2m level you'd have head height issues going from one to the other as these garden rooms have limited head height. This option just won't work for a number of reasons.

If height of the garden room is 4m or so above ground level and your neighbour’s place, I can see why he’d be pissed off. Can’t see how you wouldn’t need planning - garden room planning exemption surely only applies to ground level scenario.