Thanks for your response. I cannot seem to post in the chat window so I have switched to the question and answer format which I hope does not cause any inconvenience.
The starting point for this is the council acting via its council tax officer has discretion in these circumstances as to whether to class the (can we call it an annex for expediency here) annex for a separate council tax band. He must have regard to the level of modification the property has undergone when considering whether to charge separately or not. Your main Achilles heel is likely to be the kitchen as case law has repeatedly affirmed that the presence of a separate kitchen can be prima facie evidence of a separate dwelling as discussed below.
The below are relevant decisions by the courts which have been the result of challenges to council and valuation office decisions.
Clement –v- Brian – High Court
Bedsit units with a wc and a wash hand basis in a separate cloakroom. Bedsitting area with a fully equipped kitchen in a separate room. No shower or bath. There were communal bathing facilities. The absence of a bath or shower did not take the property (which was otherwise self contained) outside the definition of self contained so the unit was separately banded.
James V Williams
Four units, each unit had a living room, kitchen (one of the units instead had a kitchen/living room) and bedroom. All units shared a combined bathroom/wc and a separate wc compartment. However, one did have its own wc as well. It was assessed as four separate units.
Neale ex parte Moore – Court of Appeal
Consideration of status of a bed sitting room and kitchen of which the tenant had the sole use. Shared use of bathroom and wc by 13 tenants. Separate banding upheld.
You will likely need to give consideration to removing the second kitchen and then appealing the decision.To appeal initially you can write to your council, saying why you think your decision is wrong. If you appeal is rejected following a further inspection you can appeal the Valuation Tribunal using the below link. If you appeal with the kitchen still in situ case law is against you though each situation does turn on its own circumstance.
Does the above answer all your questions or is there anything I can clarify or help you with any further?