addition vs annex

addition and annex both are nouns.

Word	Noun	Adjective	Verb	Adverb
addition	Yes	No	No	No
annex	Yes	No	Yes	No

As nouns, annex is a hyponym of addition; that is, annex is a word with a more specific, narrower meaning than addition:

Other hyponyms of addition include additive, afterthought, annexe, extension, wing, attachment, elongation, extension.

addition (noun)	annex (noun)
the arithmetic operation of summing; calculating the sum of two or more numbers	an addition that extends a main building
the act of adding one thing to another
a component that is added to something to improve it
a suburban area laid out in streets and lots for a future residential area
something added to what you already have
a quantity that is added

addition (verb)	annex (verb)
	add an extra part to
	take (territory) as if by conquest