@Steven ***********
*** IF your country's embassy in Bangkok still issues a certified income affidavit.
you don't
EITHER
need 12 months of transfers of 40,000 THB from abroad into your Thai bank account,
NOR
do you need a deposit of 400,000 THB in your Thai bank account,
You would only need this income affidavit for the financial proof for an application to the 1-year Extension of Stay Permit
*** some Immigrations might want to see that you actually transfer some money into your Thai bank account, and a bankbook showing 2 or 3 months of transfers are accepted as proof