Can I extend a 90-day Non-O visa at an immigration office, or do I need to apply for a retirement visa in Thailand?

The 90-day non-O visa is essentially a retirement visa. To extend it, you must apply for an extension at a Thai immigration office after your arrival, and you will need to have 800,000 baht in a Thai bank account for at least two full months before applying. Therefore, it's advised to secure a retirement visa from your local Thai embassy before traveling and to open a bank account and transfer the required funds as soon as you arrive in Thailand.