Its in section 55:
Meaning of “development” and “new development”
(1)Subject to the following provisions of this section, in this Act, except where the context otherwise requires, “development,” means the carrying out of building, engineering, mining or other operations in, on, over or under land, or the making of any material change in the use of any buildings or other land.
There are no prescribed measurements as every case is assessed on its own merits and in context. It also depends on whether its part of another operation which has planning permission or is permitted development. So a hole for a swimming pool which is of a size that benefits from PD would be fine. Digging the same size hole just to have a hole in your garden would need planning permission. Moving a bit of earth for a flower bed would probably be ok, but moving lots of earth to change the levels of your garden or create terracing would probably need permission.
As with everything in planning, there is no easy answer, which is why I asked what you were thinking of doing?