Recording the number of storeys below ground as a range can be used where there is a lack of certainty about the exact number of storeys below ground but the surveyor is able to determine a range. This does not include the ground floor. It includes storage and mechanical plant levels only if these cover over 50% of the plan area.

where a=upper bound and b= lower bound for the range
Example: HBET:2,1  
(between 1 and 2 levels of basement)