Unless planning rules have changed, maximum height is 2 metres wherever it is (apart from 1 metre if adjoining a highway). Having said that, applying for pp is costly and long-winded and I often explained to people that if they went over the limit without planning permission then it might be the case that the authority would not consider it appropriate to intervene if; 1 nobody complained; 2 it wasn't easily visible; 3 it wasn't much over the limit. People often took a chance on the basis of what I had explained (not advised!) and none came a cropper.
If you have horrid neighbours or your proposal would be horridly unneighbourly, then you might feel it advisable to ask the planners for advice and then to follow it.