@Thomas *******
it depends on whether the child is listed as your sole custody with a family court or local amphur. If so you need 400k and quite a few trees of paperwork to apply. Here’s the list that the Phuket office needs.
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If using another office check directly with them on their requirements as they may well be different.
If the child is not in your sole custody then the best you can do is a 60 day extension for visiting Thai family.