@Jamie *******
you certainly CAN apply for a 90 day non-O visa at the immigration office straight off a 30 day visa exempt entry BUT you aren't gonna be able to do it without the full 400K baht in a thai bank account in your name only BEFORE you apply.
The reason people get the 30 day extension is because of the under review period that the immigration offices have (I think Buriram requires a minimum of 15 days left on your current stamp).
Few people can show up on a 30 day visa exempt and get their shit together fast enough document wise, funds wise to get to the immigration office while they still have enough time left on their current stamp.