I found out that flats and houses all have the same ‘use classes’ “C3 dwellinghouse” in this case there should not be ‘change of use’ if a house is split into flats so why it is necessary to ask for planning permission to split a house into flats?
No. Flats and dwelling houses are in different use-classes.
Dwelling houses are C3, and this includes HMOs up to 6 persons, but not flats, which are in a class of their own.
The distinction is important because the C3 class has pemitted development rights, while flats don't.