Just for your info, once that you get in Thailand with a tourist visa, you have to wait at least 1 month to ask for the 30 days extension at any inmigration office. I tried in 2 with same result, so dont waste your time / money as I did trying to get it in the first days of my arrival.
TLDR : Answer Summary
When entering Thailand on a tourist visa, you must wait at least one month before applying for a 30-day extension at an immigration office. However, experiences vary; some individuals have successfully requested extensions prior to the 30-day mark, and the rules may differ depending on the specific immigration office.