Learn Croatian: Prizemlje je ispod prvog kata. - The ground floor is below the first floor.

The preposition ispod (below or under) always makes the noun phrase following it take the genitive case. Since prvi kat is masculine, the adjective prvi changes to prvog and the noun kat gets the -a ending to become kata.

Are floors numbered differently in Croatian compared to American English?

Yes, Croatian uses the European floor numbering system. The street level is always prizemlje (ground floor). The floor immediately above that is prvi kat (first floor), which is equivalent to the American second floor.

Where does the word prizemlje come from?

It is built from the prefix pri- (meaning near or by) and the word zemlja (earth or ground). Together they form a neuter noun that literally means near the ground.

What if I want to say something is above the first floor instead of below it?

You would replace ispod with iznad (above). Because iznad also requires the genitive case, the rest of the sentence remains exactly the same, becoming iznad prvog kata.