I have to return to Immigration on to get my extension in about 10 days but will be near there a few days before

Is the date you are told to return precise or is it worth asking 2/3 days early and save a second journey

I always go in a few days early and have never had a problem. But that said, every office seems to have its own rules, so whatever works in Phit'lok, may or not work wherever you are. Had you included that information, somebody may have been able to offer you specific advice.

I always go in a few days early and have never had a problem. But that said, every office seems to have its own rules, so whatever works in Phit'lok, may or not work wherever you are. Had you included that information, somebody may have been able to offer you specific advice.

The OP lives in Chiang Mai.

If you are going to be near your immigration office anyway it would not hurt to check. Or you could call them to see if it is ready yet.