Earlier this year I did my extension at Chaengwattana with TVE.
Took only an hour. 40 mins for the bank book/deposit and then about 20 mins for ext application.
31,000B all in. (6k for bank book and deposit. 25k for ext.) A little expensive but another place quoted me 38k. And other places were cheaper but were located in Pattaya/Samut Prakarn.
TVE met me at CW which is on my doorstep. I plan on using them again soon.