Change of use from C3 to C4 (HMO) use with an extension - CIL liable? - Public forum - Planning Advisory Service (PAS)

We have a case where the planning permission is for – change of use from C3 to C4 (HMO) use with an extension for approximately 20 sq meters. In my understanding this is CIL Liable because this is a change of use, that is, conversion of the whole house from one use to another, therefore the whole house to be considered and the existing “in use” floor space to be discounted, however the extension creating additional floor space will be CIL Liable.

Any thoughts regarding this would be very much appreciated.  Best regards, John