After the work permit is approved and issued by the Thai Labor Department, the foreigner must apply for an extension with the Thai Immigration Office. This can be done only with 30 days before the initial visa expires.
It has to be noted that the visa process is entirely independent and unrelated to the procedure in obtaining a work permit. A valid work permit does not guarantee the right to stay in Thailand, nor does a valid business visa guarantee the right to work in Thailand.
Employees holding a work permit and appropriate visa are able to leave Thailand for whatever purpose at any time before the expiration of the visa’s validity. However, they must obtain before departing a re-entry permit which can be issued for a single occasion or unlimited use throughout one year.
This allows them to leave the country without invalidating their visa.
In case they fail to obtain it, the Visa will not be valid when they try to re-enter Thailand and they would have to repeat the whole process of visa application from the beginning.
Under current Immigration Regulations, if a foreigner stays in Thailand for a period longer than 90 days, he must notify the Immigration Division. This regulation applies even after the foreigner has obtained a One-Year Visa. This 90-day period is calculated from the date the foreigner last checked through Immigration. Failure to comply with this regulation is subject to a fine of 200 Baht per day, up to a maximum of 5,000 Baht.
Aliens should also report any changes of address or status to local police within 24 hours.