If your proposal does not qualify as Complying Development, you will need to lodge a Development Application with Council.
Secondary Dwellings are permissible under land zoned RU1, RU2, RU4, C3, R2, and R3 subject to the consent of Council. The total floor area of the secondary dwelling (excluding any area used for parking) must not exceed whichever of the following is the greater:
(a) 60 square metres,
(b) 33% of the total floor area of the principal dwelling
Secondary Dwellings are permissible under land zoned R5 in accordance with Clause 52 of Housing SEPP 2021 – Chapter 3 Diverse Housing Secondary Dwellings, subject to the consent of Council. The total floor area of the secondary dwelling (excluding any area used for parking) must not exceed 60 square metres.
The NSW Department of Planning, Industry and Environment has a useful guide to the DA process.