Who cares! No one other than you or the Postie is ever going to know its there - just build it and be done with it and stop reading convoluted planning policy!
If the planners did for some random far fetched reason ever find out, then I cant see that given your location they would have any grounds to reasonably ask you to remove it. Crack on!
Principal Elevation usually seems to be defined as the one facing "the road". This seems a bit far-fetched if the road is half a mile away across fields.
South Lanarkshire council website has the interesting definition:
"Principal elevation means the ‘front’ of the house. Most houses are built so that the ‘front’ of the house faces a road. As this is the part of the house that is seen by most members of the public, it will usually be designed to be the most important elevation or ‘principal elevation’."
I don't image many members of the public will actually see the OP's house at all
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