You can go to an agent as soon as you arrive pay
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bht and they will sort the 12 month extension out including thr money required and a bank account you can stay as long as you want,leave and return when ever you want,you need a re-entry permit these can be purchased by the agent either a single re-entry for about 1000bht or multiple re-entry for about 4000 bht or you can go to immigration and get a re-entry premit at the airport for 1000bht,this will keep you extension active .