How do you read this?

Curios learner   Wed Oct 08, 2008 10:23 am GMT
When referring to a basement floor of a building, how do you(native speakers) usually read/say it?

B1 => "Basement first floor" "First basement floor" or simply "B one"?
B2 => "Basement second floor", "Second basement floor" or simply "B two"?
G.   Wed Oct 08, 2008 12:14 pm GMT
One possibility:

B1 = Basement
B2 = Sub-basement (assuming B2 is below B1)
Kelly Wang   Wed Oct 08, 2008 2:57 pm GMT
What if the building has B3, B4, B5 and more? (like the buildings in New York or Hong Kong)
RayH   Wed Oct 08, 2008 5:09 pm GMT
<What if the building has B3, B4, B5 and more?>

Typically they are referred to by the designations assigned by the owner or architect.