Property bandings are based on property values on 1 April 1991. For properties built or converted after this date, the property band is based on similar homes in the area.
The Valuation Office Agency sets the property Council Tax bands across the country. If you think your band is wrong you can challenge it.
Making an appeal doesn't stop you having to pay council tax in the meantime. You must continue to make the payments demanded on your bill. If your appeal is successful, you'll be entitled to a refund of any overpaid tax.