in writing vs written

in writing and written both are adjectives.

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

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

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