Planning permission is needed for any ‘development’. That covers either a material change of use, or the carrying out of ‘building, engineering, mining or other operations’.
Much publicity was given to the recent case of the landowner who built a house (castle was how it was described in the press), and then concealed it from the roadway by hiding it behind bales of straw covered with a tarpaulin. After four years (the normal period for enforcement action), he then removed the straw bales and tarpaulin, to reveal his castle!