Can I extend my retirement visa in Thailand using the 12 monthly payments method if I lack the 800,000 baht in my bank account?

Your problem is that you used an agent for your existing extension so you won't have met the terms of that extension. Even if you had 800k baht in the bank its very unlikely you could do the next extension yourself. You are stuck on the agent wheel, you will have to continue to use an agent for the foreseeable future. The only way to get off the agent wheel is to leave Thailand close to the end of your current extension (without a re-entry permit) and either apply for a 90 day Non-O visa in another country or come back in visa exempt and apply for a 90 day Non-O visa at your local immigration office (by meeting their requirements) and then applying for a 12 month extension (they may or may not allow you to use monthly income at that point, but chances are they won't)

I use K Bank and they send me a detailed credit advice e-mail that shows the origin (UK) and the destination. I take 12 copies of this with 12 months statement copies. (Kasikorn will print off 6 months free - so twice yearly with a letter of confirmation on the last month). You will need copies of your bank book front and latest pages showing a deposit on the day you attend. I always did this the day before but I have seen people say their local immigration send them back to do it on the day of application). Then the relevant form with picture, passport etc.

Depends ENTIRELY on your immigration office in Thailand. They are the only people who can give the correct advice on this one, as each office has different approaches to the 65k bank transfers. I switched from OA (which did not require the 800k) via an out/in on exempt and then 90-day non-O from CW, then 12-month extension. I had started the 65k transfers whilst on OA, so I never had the 800k lump sum. However, I also never used an agent

I've been here 10 years now, 5 years on married visas and 5 years on an elite visa. Planning to switch to retirement.

Past 3 years transfering my monthly pension payments into my Thai account, same day every month. So I have 3 years bank statements and pension statements, but the local immigration insist on a Proof of income certificate from the British Embassy...

Since the British Embassy don't provide it, I'm forced to use an agent... Very frustrating. My Swedish, German and Irish friends, just pop to their embassies and get a certificate, easy peasy.

Immigration doesn't like the 12 payment visa. I have had trouble every year.

You will need the 0 for the first year before the immigration will allow you to use the 65k, unless you can find a immigration office to say you can use the 65k on your frist year

If you are currently on extension.

You will need to prove:

800k 3 months after last approval

400k always

AND proof of 12 monthly consecutive transfers of international funds.

Otherwise you'll be paying an agent.

You must have proof of 12 month of continious payments showing the money came from a foreign account otherwise it does not count

For the original non o and 1st extension you must use money in the bank method. You can switch to the 65k monthly method for your 2nd extension.