楼(lóu) - building, storey, floor in Mandarin

楼 (lóu) does two jobs. It is a multi-storey building, the thing you point at from the street, and it is a floor of one, the number you press in the lift. Which meaning applies is decided by what sits in front of it: 这栋楼 is that building, 三楼 is the third floor.

The floor-number use is where beginners meet it first, and the directional pair 楼上 and 楼下 covers upstairs and downstairs in any context. The measure word for the building is 栋 or 座, never 个.

The English-speaker trap is floor numbering. Chinese counts the way American English does, not British English: 一楼 is the ground floor, so 三楼 is the British second floor. Get this wrong and you will knock on the wrong door. A related snag is 层, which counts storeys as a quantity (二十层, twenty storeys) while 楼 names a specific one.