If you have an extension of stay but would like to travel outside Thailand for a short period then you will need to apply for a Re-Entry permit. Unlike before that this can be secured at the airport, Immigration rules have changed and you can now apply for a re-entry permit at the following:
Photocopy of passport (main page and latest entry stamp)
The current fees for re-entry permits are as follows:
1,000 Baht for single
3,800 Baht for multiple
Re-entry permits can be applied for in person or by an agent at Thai Immigration, Soi Suan Phlu, Sathorn Tai, Bangkok. If an agent is used a letter of attorney is required.
The processing time at Thai Immigration Suan Pluh, can vary from 1 hour to half a day, so if you prefer to queue, apply before lunch so you can pick it up later the same day.
A pick up can be made by courier as long as the green receipt is produced.
There is another convenient option if you are in Bangkok and have access to the internet, you can fill out out the TM-8 form and email it to firstname.lastname@example.org at least one day in advance. You must then attend the Immigration office at Suan Phlu and submit a 2-inch photo and the correct fee at counter 7.
Please remember when arriving back into Thailand, you must note the re-entry permit number on the Immigration.