I couldn’t find any decent information on this online at all – so, here is my experience…
Both articles I found about it – insisted one needed to travel to a specific office in Bangkok to extend the travel visa.
NOT TRUE. I used an immigration office in the south – so I think you can use any immigration center close to you.
1. Get a passport photo, copies of your visa – photo page, tourist visa page, and entry stamps page.
2. Withdraw 1,900 THB from the ATM.
Fill out a paper once there – mine was full front and 25% of back.
Pay, and you’re out the door. At least that’s how it was for me – took about 7 minutes.
You can extend your single or double-entry tourist visa at the end of the first 60 days, so you get 30 more days before you have to exit the country. If you have a double-entry tourist visa you can then exit the country at the end of the first 60 + 30 days and return to Thailand, spend another 60 days – and visit whatever immigration office you’re close to at the time and ask for another 30 day extension for 1,900 THB.
I think it’s up to their discretion whether they give you one – but I haven’t read any reports of tourists not getting the extension approved.