Not sure about your location but I just went to my local council to speak about this. A deck fits within "exempt" development if it complies with a number of requirements including being less than 25sqm, less than 1m from natural lie of the land and has an open roof. I asked if putting see-through polycarbonate on top of an open pergola roof still constitutes a closed-in structure and they said yes and that it requires council approval.
What is the size of your land? A lot of councils now will allow anything to fall under complying development if it is within the guidelines and your land is more than 450sqm. For example, the house behind us is currently being built without us receiving notification from council. Apparently the owners/council don't need to inform the neighbours as the block is over 450sq. My first paragraph is unlikely to apply to you if your land is over the required size (mine's under this).