Basements is a plural noun.
Basements have always been part of a house. The same as upper floors and attics are. Some may not have either but they do not become a part of it, they are a part of it.
The part of speech for this particular word is a noun.
part of speech
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
The part of speech for "unfamiliar" is an adjective.
The part of speech of "came" is a verb.
The part of speech for "twirl" is a verb.
The part of speech for "explicit" is an adjective.
The part of speech for "indefinite" is an adjective.
The part of speech for "lecturer" is a noun.