You can have multiple basement levels, which get numbered again. 1er Sousl-sol, 2eme Sous-sol etc. The ground floor might not be explicitly labeled as zero but

You can have multiple basement levels, which get numbered again. 1er Sousl-sol, 2eme Sous-sol etc. The ground floor might not be explicitly labeled as zero but the number between "one up" and "one down" is still zero.