written vs in writing

written and in writing both are adjectives.

Word	Noun	Adjective	Verb	Adverb
written	No	Yes	No	No
in writing	No	Yes	No	Yes

written (adjective)	in writing (adjective)
set down in writing in any of various ways	written or drawn or engraved
systematically collected and written down
written as for a film or play or broadcast

written (adverb)	in writing (adverb)
	as written or printed