If you leave the Kingdom of Thailand most likely any Visa that has been issued into your passport will be voided. In order to avoid this from happening, you will need to apply for a single or multiple Re – Entry permit. Apply for it BEFORE you leave at the local Immigration office or, and this is rather new, at the airport before passport control.
Make sure the airport where you will proceed through passport control is providing that service.
A single Re Entry permit will set you back ฿1000 and will allow you to leave and re-enter Thailand on the same Visa once. A multiple Re-Entry permit will allow you travel in and out as much as you like using the very same Visa and will cost ฿3800.
Calculate an additional 30 minutes to your departures timing at the airport for this.
In Chiang Mai you may apply your Re – Entry permit at the Immigration office at the Promenada Shopping Mall. The Immigration office close the Chiang Mai Airport will not issue the permit unless you extend your Visa on that very day. If you only need a Re-Entry permit they most likely will send you to Promenada Office.
As your Visa has been voided while leaving Thailand the number of your Visa has been voided, too. It has been succeeded by the number on the Re-Entry permit. Please fill in that number on your arrival card.