CDC?
Center for Disease Control? Compact Disc Case?
Or is it Complying Development Certificate?
Strictly speaking you (and your neighbours) should have some sort of enforceable agreement (like a by-law) that responsibility for maintaining and repairing any common property that is changed should be transferred to either the person changing it, or left with the owners corporation.
What did your neighbours have in the way of permission? And who fixes it if things go wrong?
Maybe none of this is common property, in which case you should first go to your local council to check and see whether your planned changes are an exempt development or a complying development.
Those links will take you to the NSW Planning Portal where there is more detail. The following is a straight lift from the latter web page:
From 1 July 2021, all Complying Development Certificate (CDC) applications must be lodged through the NSW Planning Portal.
Complying development is a fast-track approval process for straightforward residential, commercial and industrial development. Complying development generally includes larger building works than exempt development. For this reason, ‘sign off’ by a building professional (known as a certifying authority) is needed. Provided the proposal fully meets specific development standards, it can be determined by a Council or registered certifier without the need for a full development application.
But basically, start with an inquiry at your local council and then head to the NSW Planning Portal to apply online for the certification that you need.
The opinions offered in these Forum posts and replies are not intended to be taken as legal advice. Readers with serious issues should consult experienced strata lawyers.