You can apply for the "change of visa type" from the 60-days tourist visa or the visa-exempt entry, to the "90-days Non-Imm-O Retirement Visa", if you already have a Thai bank account. You however need to clarify with your Immigration office if they are willing to do the "conversion".
You need EITHER
a bank letter confirming that the 800,000 THB are sitting in the account and a proof that the money came from abroad.
OR:
You need a legalized by your embassy in Bangkok income affidavit over a monthly pension or income minimum of 65,000 THB. In case your embassy does not issue income affidavits any more, you will have to use the 800,000 THB deposit method – at least in the first year.
You need a rental contract and you will need a minimum of 15 days stay permit left on your entry stamp in order to apply.
This application costs a 2000 THB fee, the 90-days visa will be issued for 14 days "under consideration". After 14 days you visit Immigration again and pick up the stamp
And as soon as the money has seasoned in your bank account for two months, you can apply for the "1-year extended stay permit based on retirement"
(which some people call "retirement visa" but which is NOT a "visa" but a stay permit).
This application costs another 1900 THB fee
Keep in mind, in order to get any service from Immigration, you need to be properly registered in your accommodation by the TM30 registration system
Open this Immigration website and click onto number 9. It will open a .pdf listing the requirements for the “change of visa type” to the 90-days Non-Imm-O Retirement Visa
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