How many days can I extend my visa in Thailand after staying for 15 days?

The ability to extend a visa in Thailand depends on the type of visa the traveler initially entered on. Specifically, a 15-day visa on arrival is generally not extendable, and travelers from certain countries may not be able to apply for an extension at all. However, those who are visa exempt, like US citizens, can request an extension at a local immigration office, typically for a fee of 1900 THB, and should do this prior to their visa's expiry. It is advisable to visit any immigration office rather than just those in Bangkok.