add-on vs appurtenance

add-on and appurtenance both are nouns.

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

As nouns, add-on and appurtenance are synonyms defined as:

add-on and appurtenance: a supplementary component that improves capability

Other synonyms of add-on include accessory, supplement.

add-on (noun)	appurtenance (noun)
a component that is added to something to improve it	a supplementary component that improves capability
a supplementary component that improves capability	equipment consisting of miscellaneous articles needed for a particular operation or sport etc.
a software module that extends or enhances the capabilities of an existing application