When you are staying in Thailand for Retirement, Business or Family Support, you can obtain the very convenient "One-Year" visa extensions, which allow you to stay in the country continuously for as long as you comply with their conditions. However, it's important to remain aware that as soon as you leave the country, all visas are automatically cancelled, and you have to start all over when you come back. Fortunately, there is a way to leave the country without losing your hard-earned visa, by requesting the Re-Entry Permit.
What is the Re-Entry Permit
The Re-Entry Permit is not a visa, and has nothing to do with the multiple entries visas you might have heard of. It's a special stamp given by your nearest Immigration Office that will signal to the border authorities not to cancel your visa, since you are leaving only temporarily and have the intention to come back. Obviously it can only be given when you have a currently valid visa, and must be used within its validity date. There are two types available: a Single Entry permit, which allows you to go out and come back in the country only once, and a Multiple-Entry permit, which allows you to go out and come back several times within its validity date, the difference between the two being the processing fee paid to the authorities.
How to Apply
Applying for a Re-Entry Permit is very easy and is done on the spot by the authorities. You can apply at the nearest Immigration Office, and bring with you a 4x6cm ID photo and signed copies of your passport and current visa. At the Immigration Office, you will be given a TM.8 Form to fill, which takes only a few minutes, and pay a processing fee of either 1,000 THB for a Single-Entry permit, or 3,800 THB for a Multiple-Entry permit (these fees are as of July 2015). So you can do the math and see which is more cost effective depending on how many times you plan to leave the country. The officers will then stamp your passport with the Re-Entry Permit, and you can now leave the country much more conveniently, with the assurance that when you come back you won't have to restart your visa process all over again.
How to Use
Using the Re-Entry Permit is also very easy. Simply show it to the border control authorities, if it's a Single-Entry permit they will mark it as used, if it's a Multiple-Entry they will just mark the number of times used. You will get the usual Exit stamp, but you won't have your visa crossed as cancelled. When you come back, you will also get the Entry stamp, but since your previous visa is still valid, your length of stay will remain as if you hadn't left the country. Note that it does not "pause" your visa, time still runs and the expiry date of your visa remains the same when you come back. This is overall a very convenient process to know-of, as you never know when you might have to leave the country, and it would be too bad to start over the process of getting the one-year visa again, especially for those on a Business Visa, which takes a long time to obtain.