Just for information I need to "extend my stay" here every 12 months, in fact I have just done my latest today.
What I need for my extension based upon retirement in Thailand:
At least Bht 800,000.00 (US$ 23,200.00) in a Thai bank which must have been there for at least 3 months.
Copies of my passbook (bankbook) pages which prove that and the balance at the date of application.
A letter from said Thai bank to confirm the balance at the date of application.
Copies of my passport including pages containing all previous extensions (if any) and my original visa (which I am extending).
Copy of my entry card to Thailand.
Copies of my landlord's (in my case my wife's) I.D. and her "housepaper" and a declaration stating that that is the address at which I live.
A declaration (including current photograph) confirming my address and the above.
[If I'm away from that address for longer than 24 hours and still in country, I need to inform Immigration of where I am staying and also, when and if I return).