Conservatory foundations and base in heavy dry clay soil.

[Winco]

The land around my house is well drained and is made up of heavy clay which appears to be very stable, but is difficult to dig. As an example, where I put in a step, there remained a thin gap, about 12 inches deep and 3 inches wide with the step one side and clay soil the other, where I had dug further back. That was over a year ago, and the clay has not fallen into the gap, despite being walked on.

So, for a new conservatory, how deep the foundations? They say dig down until you reach stable ground. However, I can lift a patio slab and solid ground is right there!

For the conservatory base, do I really need to dig out all the clay for the starting base layer of 100mm hardcore?

Thanks