Just got back from doing an extension for visiting family on top of my extension to a SETV
My SETV extension is up on the 10th June and my new date of stay is till 5th August.
So the extension for visiting family is 60 days from the date of application and NOT added on to the other extension end date.
This was at the Korat immigration office.
TLDR : Answer Summary
The user reports that their extension for visiting family is granted for 60 days from the date of application and does not extend the current SETV expiration date. This contrasts with other users' experiences, who have noted the 60-day extension is typically added to the existing visa duration.