You're getting muddled between planning permission and legal ownership, Jasper.
In theory you can apply for planning permission on any piece of land and if you were granted it, you could put a structure up - its just that you won't own the structure because you don't own the land!
Adverse possession is a legal process that enables a person that has fenced and maintained a piece of land for over 12 years to take legal ownership of it (its slightly different between registered and unregistered land - but I'll keep it simple by sticking to just unregistered).
Until one has had occupation of the land in question for the required period, one cannot start to claim the ownership.
Does that answer it?