Yes, as LCDMAN says, without a certificate from Building Control, it is nothing more than a non-habitable shed attached to the house. So the value of the house should reflect that - don't include any value for the shed. Indeed if the original outer wall of the house has been knocked through into this 'shed', its likely to be a positive liability. It is unlikely to be constructed to adequate thermal standards, which will make the adjoining rooms cold in winter.
You should look very carefully at how the HIPS worked out the thermal values associated with this 'shed'.