@James *******
that's incorrect information.
In Chaengwattana, 1 floor above the normal immigration division there is the "e-Extension" which is for extending different types of visas including non-exempt visas.
This costs 500 baht extra but can be done online and you only go there to receive your stamp. It's way faster (no queue), way easier (mostly online) and saves a lot of headaches.
You can also just walk in and they'll assist you.