By the time I expect to retire in a year or two, I will have worked only in Thailand for over 12 years; can I use funds saved  entirely during my time working here for the 800,000 baht requirement or must I send this amount out of country and then send it back? I don't want to access my social security funds until several years later to allow it to grow.

Yes, you can.  You have evidence as to the source of your funds, not that you will need to provide such evidence. Only evidence of funds is usually enough. Had I not obtained citizenship it was the way I was planning to extend my stay.   

4 hours ago, orchidlady said:

can I use funds saved  entirely during my time working here for the 800,000 baht requirement

yes, you won't be asked to show the origin of those funds.

But mind you, one thing they do -at least in Chiang Mai- is to check how you support yourself. I am not sure whether this is official policy, but everytime at extension renewal they want to see not only my fixed deposit of 800k, but also that there is enough funds in/expenses out on my regular savings passbook.

Of course if you have saved more than enough on your thai bank account to last "until several years later", no problem.

2 hours ago, arithai12 said:

yes, you won't be asked to show the origin of those funds.

But mind you, one thing they do -at least in Chiang Mai- is to check how you support yourself. I am not sure whether this is official policy, but everytime at extension renewal they want to see not only my fixed deposit of 800k, but also that there is enough funds in/expenses out on my regular savings passbook.

Of course if you have saved more than enough on your thai bank account to last "until several years later", no problem.

Yeah.  This this type of proof is going to be troublesome for me.  I plan to bring in $10,000 USD each t rip home which is about once or twice.  Then use my ATM as needed.  I don't expect to transfer any money in at any regular intervals.  My social security and other passive income investment monies will always stay right where they are pointing now, my US financial institutions.  So if I go the income method, well, have to see how that is going to shake out in the next year as the Thais and the embassies fight that out.  I have no plans or desire to use the 800 k baht method.  That is a measurable amount of USD I prefer to keep invested (tax free of course) and I prefer to avoid filing Fincen or that 10,000 USD foreign investment report.  Not a big deal, just a preference.