you want section L-1 for Non-O visa (based on retirement or marriage to a thai)
you can make an appt for the application at this link, that way you just show up about 15 minutes before, get your documents checked by the officer, then go to the appropriate cubby for the extension.
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you want the first choice on that screen
Government Complex Ratthaprasasana Phakdi Building
then the reason you're getting an extension.
I selected "retirement"