The two structures have different foundations, different ages and different rates of movement. Where they are rigidly tied together, something has to give at the interface.
Differential movement versus subsidence
Differential movement gives a fairly straight vertical crack at the junction, often opening slightly at the top, stable in width over the seasons, and not accompanied by other distortion.
Subsidence looks different: tapering diagonal cracks away from the junction, doors and windows going out of square, cracks wider than a few millimetres and clearly growing. That pattern needs a structural opinion and usually an insurer conversation, not filler.
Monitoring before repairing
Filling a live crack simply moves the failure a few millimetres along. Where there is any doubt, marking and measuring the crack over a full seasonal cycle is cheap and tells you whether it is static.
Clay soils in parts of Kent shrink and swell noticeably with the seasons, and nearby trees amplify it, so a crack that opens in a dry summer and closes in winter is telling you something useful.
How the junction should be built
Best practice is a designed movement joint at the interface — a vertical joint with a compressible filler and a flexible sealant, or a slip tie arrangement that allows small relative movement without tearing masonry.
On a repair, that often means cutting a proper joint where one was never formed, making good the finishes each side, and accepting a visible line rather than chasing an invisible one that will crack again.
Common questions
Is a crack at my extension junction subsidence?
Usually not. Straight, stable, vertical cracking at the interface is typically differential movement. Diagonal, widening cracks with distorted openings are the ones to have assessed.
Should I just fill it?
Only once you know it is stable. Filling a moving crack gives a repair that reopens, usually within a year.
Can you form a movement joint retrospectively?
Yes. Cutting a proper joint and detailing it with a compressible filler and sealant is a normal remedial measure.
Do I need a structural engineer?
If the crack is wide, growing, diagonal or accompanied by out-of-square openings, yes — and we can arrange that assessment.