@Priti **********
Hi Priti.
I’m in the process of getting my Non-O extension at the moment, so I’m no expert but am familiar with the route I’ve taken so far. I might receive criticism but hey ho!
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Simplest to apply for the Non-O visa while outside Thailand, using the Thai e-visa site which will get linked to your local Thai Embassy. During the application you must be in the country you are applying from until the visa is issued. There are obviously questions regarding your Passport, funds in a bank account (outside Thailand) and your present location. You pay the visa fee too. They may get back to you with further requests but the whole process should likely be completed within 2-3 days if applying from a Western country. Once they are satisfied they will send a copy of the visa to your e-mail in PDF format. You should print off at least one copy of that. (I printed it off and laminated it for durability). It will define the 3 month window within which you need to enter Thailand, so don’t apply too far in advance of your planned trip.
Fill out the online TDAC information within 3 days prior to your arrival in Thailand.
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On arriving in Thailand and passing through Immigration, present your visa together with your passport. (They will have a copy linked to your passport but more rather than less is preferable.) The immigration officer will stamp your passport with your Non-O stamp and write in the date that the visa expires, 90 days later. (Know the date yourself prior to going to the desk and verify this is the correct date.)
ASAP you should obtain a Thai SIM card then visit a bank to try to open an account ASAP. The bank will inform you of any further requirements depending on the individual branch. I opened an account with K-Bank quite easily while SCB were asking for a Yellow House Book (Tabien Baan) in order to be able to open an account with them. A residence certificate from your local Immigration office might be required in lieu of a Yellow Book. (My Thai wife and I own a house).
You need to deposit the required amount of money into the account in enough time to have it deposited there for fully 2 months (not 56 days!) prior to applying for the extension, be it Marriage or Retirement. The amounts required in the account are 400,000 baht for Marriage or 800,000 baht for Retirement.
This means that the bank account has to be opened and the money deposited within the first 28 days of arriving in Thailand. Also check with your local immigration office in advance to see what their requirements are and from what time prior to the expiry of your Non-O visa that you can apply for the extension there. (Usually from 45 to 30 days before, but the money must have been in the account for the required 2 months).
(To obtain my Yellow Book I had to visit my local district office [Amphur] to find their requirements. These were extensive and apart from local things, involved getting my passport certified at the British Embassy in Bangkok [2 weeks until I could get an appointment] then translated into Thai and validated at the Thai Ministry of Foreign Affairs [MFA] office, with further requirements from them! Anyway, I did receive it together with a pink Thai I.D. Card eventually. You’ll likely not be required to go through this for your extension of stay, but in the long term having them will simplify your life.)