Subsidence panic! Step coming away from front of house

[concretepanic]

I just went outside and moved some watering cans and saw that the concrete step that goes up to our house has come away from the wall by about 6 centimetres.

It's hard to explain but there's a porch and the step goes along the porch and continues under the bay. It now slopes down so there is a gap between the house and the step.

We are on a hill in an area with lots of subsistence but the house is fine, a few minor cracks in the plasterwork but nothing particularly worrying.

What could it be?

It’s probably the steps subsiding rather than the house. Our house is underpinned and the steps added later. They move all the time back and forth (a gap opens and then it closes) and need replacing now. It’s common if you have clay soil.