Original Dwellinghouse for PD after extension - Public forum - Planning Advisory Service (PAS)

The property in question was modified in 1975 (built pre 1948) - around half demolished and rebuilt on approximately the same footprint. The District Council say that as this is not the original dwellinghouse, PD doesn't apply - even though new rules are based on distances, and the walls are in the same place. Also seems that they won't allow PD from the parts of the house which still stand from the 1800s either.... Anyone got any views or advice on this one?? Thanks