@Christian *********
, as people pointed out it's your misuse of terminology that is confusing you. IF you don't get a tourist vsia from a thai consulate in another country BEFORE you come here you will be entering thailand on a visa exempt entry NOT on a visa on arrival.
You get an extension of stay at the thai immigration office NOT at a thai embassy (because there are no thai embassies in thailand, ;) Just like there is no US embassy inside the USA)
It is going to totally depend on the immigration office you use for the 30 day extension once you're here. Some will do it right away, some want you to have 15 days or less, etc.