Nowadays music for the bass clarinet in Bb is written in treble clef. The sound is a major ninth lower than written.
This type of notation is known as “French notation” and is the easiest for a clarinetist to read because it is the one most commonly used for all clarinets: what the written voice actually refers to is the fingering, which is the same on all clarinets; the pitch that sounds depends on what pitch the instrument is tuned to (the sound of the instrument is transposed by whichever interval is required by its tuning).
Other types of notation
In the past other types of notation were used instead of French notation.
Notation for the bass clarinet in Bb in bass clef; sounds a major second lower than written (the bass clarinet in A sounds a minor third lower than written).
The highest notes are written in treble clef; the sound is also a major second (minor third) lower than written. Richard Wagner and Richard Strauss use this type of notation.
“German” notation in bass clef (sounds a major second lower), “French” in treble clef (sounds a ninth lower). Used by Nikolai Rimsky-Korsakov, for example.