You will not need approval if the proposed building alterations meet all of the relevant development standards for exempt development.
Specific information about minor building alterations can be accessed here:
About Exempt Development
If the proposal doesn't qualify as Exempt Development, you may be able to apply through Complying Development or a Development Application.
A Complying Development Certificate may be issued for residential building alterations if the proposal meets all of the relevant development standards.
Check if your proposal is a complying development at:
If your proposal does not meet all of the standards you must lodge a Development Application.
If your proposal does not qualify as Complying Development, you will need to lodge a Development Application with Council.
The NSW Department of Planning, Industry and Environment has a useful guide to the DA process.