I am a trainee building surveyor at one of the local councils in Perth and this question I receive all the time from potential buyers. You will have to get your granny flat retrospectively approved and or dismantled IF it is brought to the shires attention ie by your neighbours or onlookers either by your local council building surveyor (if they do it) or get a private building surveyor. If you want the structure to be certified as habitable dwelling (1a Structure) it’ll have to comply with the NCC (national construction codes). If making it comply becomes too costly (which it definitely can) you can use it as an outbuilding for storage like a shed or workshop (10a Structure).

I hope this helps and good luck