Most likely to be minor "settlement" Sandy. Very different from "subsidence". Subsidence is usually a major(ish) condition where the foundations have have moved because the ground under the footings has lost its capacity to carry the load. Technical stuff such as "frost heave"; "clay heave", trees; water washing the ground away...... some of the likely causes.
Settlement is where a load has, how can I put this? ..... found its final resting place. Much, much smaller than subsidence. A millimetre settlement can easily give a visible crack in the render.
Construction is not quite the exact science that many people think it is. This commonly occurs even after all best practices have been followed religiously.
Old practice was to simply bang two lumps of masonry together (house & extension), and leave it at that.
Better practice is to physically "stop" render on the extension with a deliberate butt joint known as a crack line/crack joint/expansion joint. In a way, you're building-in a crack ........ so that it doesn't have to occur anywhere else. It's discreet and unobtrusive.
Sorry about the long rattle, but I thought you might like to know.
Epilogue (sorry)
Probably minor settlement.
If it's timber frame... probably minor shrinkage.
If you're worried, a Surveyor could bond glass "tell-tales" across the crack to indicate major movement if it exists. I would guess that it would only be then that an Insurance Company would take notice.