Actually the main immigration office at the government complex out on Chaengwattana Road is NOT where you go to get a 30 day extension on a visa exempt entry ๐ฎ ๐
You go to the immigration office located at IT Square Laksi Plaza. That's the ONLY office in Bangkok to get 30 day extensions on visa exempt entries.
It's here
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You also need to have a TM30 filed by your hotel or where ever you stay showing you're registered as staying in Bangkok. There are copy/photo shops next to the immigration office.