Can I open a bank account in Thailand on a Non-O visa for a retirement extension?

You might go to the Bangkok Bank website. Go to the accounts sections and scroll down to the bottom. There's a link to contact them. Tell them you will come to Bangkok with a Non-O and will need to open an account to transfer in your 800,000thb required for your Retirement Visa. Ask them for their current requirements. I would suggest you open two accounts. One for the 800k Retirement requirement, and the other for your regular spending. This allows you to keep your 800k account clean and easy to look through for immigration. They like it when you make it easy for them. To do this, you will likely need to ask the immigration office for two (2) Certificates of Residence (one to open each account). The 800k can be in a type of savings (orange book) that gets a little better interest than the regular savings account (blue book) You might mention you want to open these two accounts when you contact them in order to confirm that won't be an issue for them. Good luck.

Almost impossible without a Non O visa, and even then you might meet some difficulties. Some branches also demand a housebook or that you’ve done your first year extension of stay to get an account, but you will succeed with a Non O if you visit several branches.

Get your non-O visa (90 days) in your country. This is a visa and try to get a bank account.

2 years ago for Bangkok Bank this was enough but want to see my driver license (national EU).

SCB want to see a certificate of residence. Some immigration edit it only after the first 90 day report.

Get the 90 day Non-O visa from the thai consulate in your country before you come.

That will be better than showing up free stamp and trying for a bank account.

once you get an account open get 800K transferred in so it can season the 2 months required to get the year extension (y)