Just wanted to confirm when you apply for an extension, that extension starts not on the day you applied for extension, but on the day your visa runs out? I don't think you need this info to answer but Im from U.S.,this is my first time ever (been here one month) in Thailand and my SETV runs out at the end of August. I just need another 30 days and already have my plane tickets booked back to the U.S. I plan on applying for extension in Ayutthaya.
TLDR : Answer Summary
When applying for a visa extension in Thailand, the extension period begins on the day your current visa or entry stamp expires, not on the day you submit the application. In this case, the user, a U.S. citizen with a SETV visa expiring at the end of August, aims to get an additional 30 days before returning to the U.S.