surface area vs extent

surface area and extent both are nouns.

Word	Noun	Adjective	Verb	Adverb
surface area	Yes	No	No	No
extent	Yes	No	No	No

As nouns, extent is a hypernym of surface area; that is, extent is a word with a broader meaning than surface area:

surface area: the extent of a 2-dimensional surface enclosed within a boundary
extent: the distance or area or volume over which something extends

surface area (noun)	extent (noun)
the extent of a 2-dimensional surface enclosed within a boundary	the distance or area or volume over which something extends
	the point or degree to which something extends