I will be arriving in Bangkok mid-March on a 30-day visa exempt and want to extend it for another 30 days. When can I start the process and how long does it take to receive the extension (same day?). Thanks to all in the forum. 

I'm going to move this to the visas forum for more directed responses.

 

normally extension are applied for in the last 7 days of the permission to stay, 1,900 baht at local immigration office.

 

possible drawbacks are, airline may require a flight out of Thailand within 30 days of you arriving. and once in Thailand local immigration may require a tm 30 form to have been submitted, by the owner of the place you are staying, this doesn't apply at all immigration offices.

Application takes less than one hour depending on how busy the office is.

if you are arriving mid-march and your visa exempt entry will end mid april, you should probably try and get the extension completed just before the end of march.  the songkran holidays will be mid april and immigration will be closed for a few days for those holidays.  i know chaeng wattana (CW) in bangkok can be quite busy before/after a holiday.  i've done extensions on visa exempt entries at CW and the time required has ranged from 45 minutes to 6 hours.  they close for lunch so you lose that hour if you haven't recvd your passport back by then.

Just in case the airline gives you greef I would get a visa in your home country then you would have 60 days on arrival.

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