I am aware of cases where planning permission wasn't required for a granny-flat, so the above may not always be relevant.
What LA area was that? Most of them include 'This exemption does not apply to the subdivision of a house into flats or granny flats. Planning permission must be obtained for such works.' in their explanation of Exempted Development. Kildare go so far as to say any conversion that is not for use as part of the main house such as a granny flat/ self-contained unit does not fall under the exempted development conditions of the Planning & Development Regulations 2001