Unfortunately its not always as simple as that. This is the wording fromt the technical guidance
In most cases, the principal elevation will be that part of the house which fronts (directly or at an angle) the main highway serving the house (the main highway will be the one that sets the postcode for the house concerned). It will usually contain the main architectural features such as main bay windows or a porch serving the main
entrance to the house. Usually, but not exclusively, the principal elevation will be what is understood to be the front of the house.
There will only be one principal elevation on a house. Where there are two elevations which may have the character of a principal elevation (for example, on a corner plot), a view will need to be taken as to which of these forms the principal elevation. Note, however, that in such cases the second elevation will also be subject to the restrictions under Class A if it is a side elevation and fronts a highway.
In your case, I am guessing that the main architectural features do not face the road (not all properties face the road) and without seeing the elevations of the house itself it is difficult to form an opinion. But just try to apply some common sense. Does the elevation that faces the road look like the back of the house? Does the elevation that faces away from the road look like the front of the house? What would most people say if asked? Planning Officers arent trained to define a 'Principal Elevation', that was something invented by the Government when it introduced all this new permitted development, its generally a matter of judgement. The fact that the door is on the back doesnt in itself make that the principal elevation. If you want some more speficic advice I would suggest attaching some photos of the house here and see what kind of response you get.
Even if you got a response from your local council it is only likely to be officer advice and not binding on any future decision. The only way to get a definitive answer would be to submit a Lawful Development Certificate, but then why use a nutcracker when a sledgehammer is available :-)
Hope this helps
