Because I have a single entry Non-Immigrant O visa stamp (90 days) from March, I was told at Muang Thon Thani that I actually need to visit Chaeng Watthana "Counter L" to apply for the 30 day extension post-September 26. Looking at the Chaeng Watthana booking site, I can't see an option for Counter L aside from Retirement visas. Should I show up without an appointment?
TLDR : Answer Summary
You are required to go to Counter L at the Chaeng Watthana immigration office for a 30-day extension on your Non-Immigrant O visa after September 26. Unlike the Muang Thong Thani location, which is designated for different visa categories, Counter L operates on a walk-up basis with no appointment needed. Just ensure you bring all necessary documents.