add-on vs annexe

add-on and annexe both are nouns.

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

As nouns, annexe is a hyponym of add-on; that is, annexe is a word with a more specific, narrower meaning than add-on:

Other hyponyms of add-on include additive, afterthought, annex, extension, wing, attachment, elongation, extension.

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