It is indeed for the extension.is it only for extension, or for initial application to this type of visa as well? What about bank statements from abroad, they don't consider it?
Before applying for 1 year extension, you need first to get a 90-day non-immigrant O visa that you can get in any Thai embassy/consulate abroad. Financial requirements will vary according to each consulate (e.g.: "Applicant’s bank statement showing at least THB 400,000 or equivalent in other currencies for the past 2 months", "Applicant's recent bank statement during the past 6 months", ... ).
It is also possible to convert your current visa or visa exemption into a non-immigrant O visa within Thailand.
Financial requirement (Bangkok Chaeng Wattana immigration office): need to show 400K THB in a Thai bank account on the day you apply. You need at least 15 remaining days on your visa or visa exempt stamp to apply.