You would certainly require planning permission (much like a large satalite dish).
However you'd never get 20 feet of it above the ground.
More realistically you'd cut off the first 4 foot (the tapered nosecone) then build a frame within the next 2-3 feet section (to stop it collapsing) and set that in a foundation. For a 4 foot diameter I'd guess at least a 7 foot diameter hole would be required before lowering it in, then straightening it up before pouring the concrete.
Overall you'd get about 13-odd feet sticking out.
However this one is destined to become the MKB BBQ MK II so we're already on the look-out for an old dual-axle trailer as a base.