With regards to my forthcoming one year extension based on retirement,  the required  funds will have been in my Bangkok bank account for 3 months, about 1 week prior to the present visa expiry date.  Does this mean I cannot make application for renewal any earlier than that date?   Ideally, I'd like to have the application completed sooner, so will I have to obtain a letter from my Embassy confirming income if I do wish the process completed early?

Any advice / experiences much appreciated.
 

All offices will allow applications 30 days early before expiration of current permission to stay. Some even longer. But the seasoning rule is the same and is based on the application date not the expiration date.

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The 800k baht must be in the bank for 3 months on the date you apply for the extension of stay (not a visa).

Unless you can show 65k baht by way of the income letter it will not help at most offices since they want the funds to be in the bank for 3 months using the combination of money in the bank and income,

What seems not to be generally known is that senior immigration officials are allowed to use their discretion to waive funds seasoning. They are usually reluctant to do this (at least more than extremely rarely) because doing it often can be regarded as abusing their discretion. Also, the official who initially checks your documents obviously does not have the power. If you have a really good reason, you could ask the official you first speak to if you could request waiving of seasoning from the official in charge.

Why not just wait for the three months to elapse as you have 7 days left after that? I did so with my first extension based on marriage.