Apart from exempt properties, each non-domestic property has a rateable value which is set by the Valuation Office Agency (VOA), an agency of Her Majesty’s Revenue and Customs. They draw up and maintain a full list of all rateable values. The rateable value of your property is shown on the front of your bill. This broadly represents the yearly rent the property could have been let for on the open market on a particular date. For the revaluation that came into effect on 1 April 2017, this date was set as 1 April 2015.
The VOA may alter the value if circumstances change. The ratepayer (and certain others who have an interest in the property) can appeal against the value shown in the list if they believe it is wrong.