My 90-day check-in was due recently but I had no idea where the immigration office was in Bangkok. I did a search but was still unsure. After some more digging I found that the office had recently moved, explaining the conflicting information on the web. It's now inconveniently located in the north of the city away from anything expats or tourists are likely to frequent.
Tony Joh of Thai-FAQ has a great video on the topic:
But there are a few things Tony has skipped. First the immigration office hours:
Monday - Friday: 08:30–16:30
Closed for Lunch: 12:00–13:00
Closed Saturday, Sunday and Public Holidays
To give you some idea how out-of-the-way the new immigration office is, the blue marker is the Mo Chit BTS station and the red is Bangkok Immigration.
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Tony recommends taking a taxi in his video but as you can see on the map, it's very far. A taxi will cost 100 baht up. The better option is taking a van, like the ones pictured below. When you exit the BTS station on the Jatujak park side, you'll see a row of taxis, buses and vans. Find the van ต10 "จตุจักร-ปากเกร็ด," it costs 25 baht.
The van will drop you at the entrance to the government compound where a shuttle bus will be waiting to carry you to the Bangkok immigration building.