Separate entrances/partition off a house - DA needed?

If I do something like partitioning half of my house off, add a separate entrance and effectively making it as if there is a separate residence within an established home, is a DA required? Going to share the same meters for electricity and water.

Or am I be better off plonking a granny's flat as that's tried and tested?

This is in NSW. Thanks :)
is it for family or are you looking at renting out for cash?

Do it discretely, add additional door on the side of the house and it could be a "teenage retreat"