You have to have 400K baht in a Thai bank account in your name only for at least two months as of application day for a 1-year extension based on a spouse to a Thai female.
Funds of 800k baht are for an extension based on retirement, not for marriage to a Thai.
, you have to apply for initial visa first. Those are requirements for a 1-year extension after initial visa. You need to check those for initial in-country Non-O visa at your local immigration office.
Thank you for the report. That is the same financial requirement of the banked money method for a 1-year extension based on retirement/being over 50, the same as in a police order.
This is about the Chiang Mai office.
Please check with your local immigration office for their detailed requirements as some offices might have a longer seasoning period.