What documents are needed for a Non-O visa extension in Bangkok, specifically regarding bank statements and letters?

You need;

- Standard bank letter (the one that has your name, account and balance the date the letter was issue)

- If you are getting another year extension from one you're on now, you need the year transaction detail report printed out by the bank for the previous 12 months

Whereas

- If you are getting the first yearly extension on a 90 day Non-O visa you just need the 3 month detail transaction print out

- Update the bank book the date you apply for the extension to show the balance