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If the use of the apartment is ancillary to that of the main house, then you might not need permission to convert. However, many councils take the view that self-contained annexes and granny-flats are ‘tantamount to a new dwelling’ and treat them accordingly. There’s nothing in new legislation that affects this. If you want the accommodation to let out or to use as a holiday let, you will certainly need permission. Seek pre-application advice from the council, or get professional advice from a planning consultant as issues around such outbuildings can be complex.