What bank statement and documentation are required for a NON-O 1 year extension based on marriage in Thailand?

I took a sample letter to my Kasikorn branch. Just showed it to the staff and within 15 minutes got the letter and a 6 month transaction printed out.

My immigration office accepts the said bank letter and transaction record dated 7 days (doesn't specify if working or calendar days) after it's printed to your appointment date. Eg. Appointment 30 Oct, then bank documents cannot be dated older than 23 Oct.

Also on the day before you go to the immigration office; you would need to make a small transaction i.e. withdraw Bht100 from the ATM and update your bank passbook to show the minimum Bht400K is still there when you submit your application.

I'm witb KTB and I just went into the branch and they did it for me there and then, took about 15 minutes from start to finish

It would help if you could advise what office you are registered.

you need to have the 'standard' bank letter written (most offices accept that dated within 7 days BUT check with your immigration office)

AND

You need to have the bank book updated on the date you apply to show the 400K baht is still in the account and has been in there for 2 months

I'd also ask them to print out a detail transaction history for the last 6 months (in case you didn't update your bank book regularly)

This is what the 'standard' bank letter looks like (this example is bangkok bank but they're almost all the same, just your name, account number, current balance and the date they wrote the letter