When does the 30-day extension take effect for a tourist visa in Thailand applied before expiration?

If you apply for a 30-day extension on your tourist visa in Thailand before it expires, the extension will take effect from the expiration date of your original entry stamp. In your case, if your current tourist visa expires on August 1, the extension will start from that date, not from the date of application (July 9). Most immigration offices typically allow applications for extensions up to 14 days before the visa expires.