When can I apply for a 30-day extension on my tourist visa in Thailand?

You can apply for a 30-day extension of your tourist visa at most immigration offices when you have 30 days or less remaining on your current visa. The extension will be added to your existing expiration date, so you won't lose any time by applying early. Some specific offices like in Jomtien or Samui allow applications when there is just one week left. It's advisable to check directly with the office you plan to visit, as procedures can vary.