This is a very important and common misconception. A building is either listed or it’s not. For example, people would often say ‘only the front facade is listed’, which is nonsense. It may well be that the conservation officers have earmarked certain features as being of particular importance and that permission to alter other parts may well be granted, but if you are making any alterations to a listed building – be it only to the 1970s extension, the inside, or even just the ugly bits – you will be committing a criminal offence if you proceed without Listed Building Consent (LBC). Please note that LBC is separate from planning permission. Often the two applications will go together and be processed in parallel, but they are judged on different criteria. In some cases you may need LBC but not need planning permission.
Many people do not realise that listed buildings do have permitted development rights, which means that much can be done with the need for a planning application – but that is not to say that you will not need LBC. The good thing is that if you have LBC, your works may well be VAT free.