I am about to get an extension of stay on my non Imm B visa, I believe I also pay for a multi entry visa, so 1900 Baht for extension and 3800 for re-entry visa.

With the extension do I still need to go to immigration every 90 days and extend for another 90 days? also does the re-entry visa last for the full year or only for 90 days and I have to renew that every 90 days?

Thanks

An extension of stay allows you to stay in Thailand for 1 year, without the need to leave Thailand. So no 90 day border runs, instead you have to report your address every 90 days, which can be done by mail.

The re-entry permit is valid for the duration of your current permission to stay, so for 1 year. It ends when your next extension of stay starts and is only needed if you travel outside of Thailand and want to come back in.

A normal B entry extension of stay will be one year so once that is stamped in passport you obtain the matching one year multi re-entry permit (don't do during any under review period as would only cover that time).

You have to make address reports if you stay in Thailand longer than 90 days but most places accept in person, second party or mail for this.

Thanks for the information, that has cleared up my questions.