Ok, so this is strange. I decided to go to the Chonburi Immigration office (servicing Pattaya), for a 30 day extension.
They required hotel reservation with some other supporting documentation (business card from the hotel or something). Or you can use a private residence with some other supporting documentation. I had none of these as I was using AirBNB (turns out to be a big no-no).
As I was waiting in line to do the 'check-in' part of the extension, the guy in front of me was saying how once, he had done the same thing. However, because he didn't 'check-in' within 10 days (or some small window) he was fined 1600 Baht.
I didn't stay to complete the process. Based on my limited experience, unless you need to- avoid the Chonburi Immigration Office.
Anybody else agree (or disagree)?
TLDR : Answer Summary
The user shares their experience of attempting to extend their visa at the Chonburi Immigration Office, where they encountered strict requirements for hotel reservations and supporting documents, specifically mentioning that using AirBNB could lead to complications. They noted a fellow applicant's fine for not checking in within a specified period, suggesting a cautious approach to the office's regulations. Other users provided additional context about the importance of filing a TM30 form and recommended considering more 'user-friendly' immigration offices, like those in Bangkok.