annex vs add-on

annex and add-on both are nouns.

Word	Noun	Adjective	Verb	Adverb
annex	Yes	No	Yes	No
add-on	Yes	No	No	No

As nouns, add-on is a hypernym of annex; that is, add-on is a word with a broader meaning than annex:

Other hypernyms of annex include addition, improver.

annex (noun)	add-on (noun)
an addition that extends a main building	a component that is added to something to improve it
	a supplementary component that improves capability
	a software module that extends or enhances the capabilities of an existing application

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