I did the same extension but got under consideration stamp until 23th of November (today). Went back to immigration just to hear that it's not approved yet, please come back Friday.
I called my lawyer. She said this type of extension is valid from the day of application. I am not sure if she is right. Did some people get this extension (visit a family) from 27th of September?
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.