How do you apply for a DA without informing the other owners? Is this NSW? If so, the DA is supposed to be announced to all and sundry via a notice on the outside of the building. If it isn’t, you have grounds to appeal to Council that the DA was improperly advertised during the exhibition period.
I also don’t know what you mean by “…despite the owner, who is on the SC and is not an owner…”. Too many owners, SCs and acronyms to make sense of your post. SC can mean both Strata Chair and Strata Committee. It’s confusing.