What documents do I need for a 12-month extension of a NON-O visa in Thailand?

If your not married i would leave her out of the equation

Most Important is that you must Have 0 THB in your account

Updated passbook. I just put in 100b on the day. 12 month statement..to show you've kept the balance above 400,000 etc

3 months of bank statements if its your 1st extension, otherwise 12 months, letter from the bank addressed to immigration certifying the money in your account. Both of these can be up to 7 days old, but your bank book needs updating on the day you go to apply

and on Immigration, you will need to sign two more forms, one of these is STM2 (an acknowledgement of having seen the rules regarding overstay) and the other form I don't remember. Nothing big

and I forgot: 8. updated on the same day bankbook and you will need to make a small withdrawal at the bank’s ATM and you need to keep the receipt slip, Immigration might want to see it

This would be a typical list of requirements, this one is from Bangkok Chaeng Wattana Immigration.

1. Your passport with the current Non-O visa and arrival stamp.

2. Filled-out application form for extension of stay (often form TM.7 for extension of temporary stay).

3. Recent passport-style photo(s) (typically 4x6 cm) taken within the last 6 months.

4. Copy of the passport biodata page and the visa/entry stamp page(s). Some offices may ask for originals to check.

5. Proof of address/registration in Thailand (for example your current residence address, TM.30 registration if required).

6. Financial proof to meet the minimum amount requirement:

EITHER a bank deposit account in Thailand of at least 800,000 THB

(maintained for at least 2 months before application)

OR a monthly income of at least 65,000 THB (for retirees) Immigration Thailand

Or a combination of deposit + income amounting to at least 800,000 THB

For citizens of countries whose embassies don’t issue Income affidavits, there is no way around the 800K deposit, at least in the first year

7. Bank letter of guarantee (from your Thai bank) confirming the deposit (when using deposit method) plus recent bank statements.

All copies need to get signed by you in blue ink

For my renewal, At the immigration office by the arena, all I took with me was my girlfriend, lease, and my passport, and the bank letter issued the day of.

The office there handled the photos and copies. Very easy compared to Pattaya.