Part L2B Regulations:  Relevant to Existing Buildings other than dwellings

Extensions to existing commercial buildings.

The construction of an extension would be treated as a New Build commercial building if the Total Useful Floor Area of the existing building is increased by 25% or more.

If so, an Air Leakage Test will need to be carried out in accordance with L2A.

Ordinarily it is just the new extension that is tested but you should liaise with Building Control to confirm this