ground level vs storey

ground level and storey both are nouns.

Word	Noun	Adjective	Verb	Adverb
ground level	Yes	No	No	No
storey	Yes	No	No	No

As nouns, storey is a hypernym of ground level; that is, storey is a word with a broader meaning than ground level:

ground level: the floor of a building that is at or nearest to the level of the ground around the building
storey: a structure consisting of a room or set of rooms at a single position along a vertical scale

Other hypernyms of ground level include floor, level, story.

ground level (noun)	storey (noun)
the floor of a building that is at or nearest to the level of the ground around the building	a structure consisting of a room or set of rooms at a single position along a vertical scale
the height of the ground on which something stands