I am soon to apply for my annual one year extension of stay based on marriage. I have a Non-Imm B visa as I also have my own company and work permit in Thailand. All my paperwork should be in order, but I have one question regarding showing the required 400.000 THB income/funds. As I pay salary tax for a monthly income of 48.000 THB per month (same as 576.000 THB per year), do I really need to show 400.000 THB in a Thai bank account? It should be enough to show official receipt of the taxes paid? Anybody can enlighten me, please!

You need to show a minimum income of 40K baht OR 400K in the bank so you don't need to show any money in the bank. Proof of your Thai income is enough.

Thanks, Elviajero. Just as I assumed it was. Just that somebody who had been to see a lawyer told me otherwise, that I needed to show money in the bank as well. But that's all cleared up now. Again, thanks!

If you are applying for 'support thai wife' then this is a reasonable reference

http://www.samutprakanimmigration.go.th/list-of-documents-for-support-thai-wife/

In Samut Prakarn you will also need to provide a copy of every page in your passport which isn't a blank page...print it across a portrait A4 (not length ways as I did).
Two copies of everything.

They want it presented in a certain order. There's no point trying to get it perfect before you arrive as it seems to change every year / at the whim of the officer.