suffix
-story building; -storey building
Suffix attached to a number to indicate the number of floors of a building, e.g. 3階建て (three-story building).
あのビルは10階建てです。
That building is a ten-story building.
2階建ての家に住んでいます。
I live in a two-story house.
階 is the counter for floors, while 階建て is a suffix meaning '-story building'.
Compound of 階 (kai, 'floor, story') + 建て (date, 'building, construction').