@Farn ******
VERY BAD advice! The problem is the airline, if your return flight is outside of the first 30 days, they can ask for a one-way flight ticket out of Thailand within the first 30 days. This means added costs on top of the 1900 THB for the 30 days extension. A 60-days tourist visa is not only cheaper but also much more convenient, you won't lose half a day on the Immigration office and you don't need the onward travel proof for the airline