I've heard one or two offices in the country do things differently, however, the general rule is that during any 90 day entry, when you have less than
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(depending on office) days remaining, you can apply for the 1 year extension. There is quite a bit of paperwork required, so best to visit your local immigration office in advance to get a copy of their printed list of requirements and confirm their timing. This will include >B400,000 in a bank account in only the name of the foreign spouse, seasoned for 2+ months, photos of couple at home, marriage certificates, etc.
After applying for the extension there is a waiting period of a few weeks, during which time immigration will normally conduct a home visit, and then you return to the immigration office to get the 1 year extension stamped into passport.
After getting the extension, you can then get a re-entry / multiple re-entry that will allow travel in and out of the country.
There is also the option to go to your local immigration office in Thailand and get the 1 year extension on the visa. Avoids the need for overseas travel every 90 days, but paperwork is a little annoying to complete
I was previously stamped in for 1 year and 60 days instead of 60 days. only noticed when applying for an extension, went to local immigration to have it corrected, but the correction was still wrong, had to be crossed out again and done 3rd time
Last year I applied for a 30 day extension, but was stamped in my passport for 1 year and 30 days. (Officer set the wrong year on the date stamp). I thought about staying for longer than 30 days, but decided to go back to the same immigration office, and they corrected the stamp to 30 days.
thank you for the detailed response. Due to our travel dates it won't work on my current entry stamp, and after returning I won't have 45 to run down visa plus 30 under consideration (75 days in Thailand plus spare days).
From what I've read of other people's experiences of the 1 year marriage extension maybe I should be glad I don't have to do it this year 🙂.