I’m not 100 percent sure that the wall is common property. have a look at the original plan and see if it is a thick black line (which means it is).
This in to something that should be rushed into. I’m pretty sure you will need a DA from your local council (and don’t fall for the “complying development” nonsense – builders hand out certificates as if they were lollies).
It’s up to the owner who wants to make the changes to make the running and your committee shouldn’t approve anything unless there are guarantees tied to the lot – ie, if anything goes wrong, subsequent owners of that lo have to pay for it.
Talk to YOUR CHOICE of strata lawyer at the applicant’s expense. Structure your approval so that the applicant is fully liable now and in the future and that the choice of surveyors is yours. If they refuse, simply refuse to give them permission and, if necessary, seek interim orders at NCAT to prevent them doing the work.