walk-up apartment vs apartment

walk-up apartment and apartment both are nouns.

Word	Noun	Adjective	Verb	Adverb
walk-up apartment	Yes	No	No	No
apartment	Yes	No	No	No

As nouns, apartment is a hypernym of walk-up apartment; that is, apartment is a word with a broader meaning than walk-up apartment:

Other hypernyms of walk-up apartment include flat.

walk-up apartment (noun)	apartment (noun)
an apartment in a building without an elevator	a suite of rooms usually on one floor of an apartment house