Not illegal unless he intends someone to live in it, then it can only be inhabited for 30 days without applying for planning permision and paying council tax on it. After 30 days inhabitation (I think that is the time allowed) it becomes another dwelling and is classed as such.
There was asimilar thread a while back from someone who put a caravan in the garden...
Oh gord. Another person who insists on multiple postings. Just wasted my time posting against the other entry without knowing that Eddie had covered it well here. Please don't do it.