My understanding is if you extend your house but stay then it shouldn't get rebanded... The council might try but I think if you appeal against it then you would win, but the next occupant will get the higher rate once the house has sold !
I've extended my house by removing the roof and heightening it, giving myself a complete extra level (basically 50% house increase) plus I've added a very large conservatory. My rates are still in the 'E' bracket, as they were when originally banded when council tax came in.
Just in case I have got this wrong and should have gone up in bands, I'm not telling you where I live, then you can't all grass me up !!!!
Plus as I said, I get 25% off for being the only adult here...... Missed her when she first left, but hey, £600 a year deduction so not all bad
(what age women will the ZED attract?????)