I will be applying for my first extension of stay based on retirement shortly, having worked in Thailand for many years on a Non-Imm B visa with work permit.

I will be applying on the basis of 800k in a Thai bank.

My bank claims they have never issued a letter to Immigration Department before, and are unsure what to put in it. Is there a format letter somewhere showing what Immigration Dept expect to see?

Help would be appreciated.

Every bank should have a template on their computer.

From memory it needs to state your name and address. Account number.

Balance at the time of writing. The date.

Signed by an official and stamped with the official bank rubber stamp.

You also need the bank book updated to show the same balance as the letter.

Does the letter need to confirm the length of time the money has been in the account, or can Immigration see from the bank passbook?

This link has sample posted of their K-bank account.

The bank book will show the dates when the money was deposited.

Does the letter need to confirm the length of time the money has been in the account, or can Immigration see from the bank passbook?

They will check passbook for that info.

My Bangkok Bank letter gives date account was opened and balance in account on date of letter. Make sure they know letter needs to be on bank letterhead.

Lite Beer & Ballbreaker - thanks a lot for your help and advice

Is a bank letter really required, would a bank book updated on the day of application not suffice?

Is a bank letter really required, would a bank book updated on the day of application not suffice?

Nope it has to be an official letter from the bank.

As you appear to be in Bangkok if you have a normal Thai bank they will have a branch office one floor down from Immigration at Chiang Watanna and am sure they can provide the required letter. There is a normal charge of 1-200 baht. You will also need to have your passbook updated to current date.