walk-up vs apartment building

walk-up and apartment building both are nouns.

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

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

Other hypernyms of walk-up include apartment house.

walk-up (noun)	apartment building (noun)
an apartment building without an elevator	a building that is divided into apartments
an apartment in a building without an elevator

walk-up (adjective)	apartment building (adjective)
a building with no elevator