@Bob ********
The O/P is asking about the funds required for an extension based on marriage (that's 400K baht) you're confusing it with one based on retirement (that's 800K baht) There are totally different seasoning requirements on the two extensions
Marriage is 400K baht in a thai bank account in your name only for 2 months before you APPLY for your extension (AND some offices want you to leave the money in the account until the under consideration stamp runs out too)
Retirement is 800K baht in a thai bank account in your many only for 2 months before you APPLY for your extension, the money needs to stay in the account for 3 months after you get the extension AND then the balance can't go below 400K baht the rest of the year