HMRC require companies to revalue properties which fall under ATED rules to be revalued every five years from 1 April 2012, this includes properties that are valued at £500,000 or under.
The re-valuation date is 1 April 2022 and you can either obtain a professional valuation or work out the value yourself, but if you do this it must be on an open market willing buyer, willing seller basis and be a specific amount. The new valuations will then be used for five years from 2023/24.