Visit HM Land Registry to find the sold price of a property in the UK. You can search property transactions using a partial or full address. Depending on how much information you have, one or hundreds of results will be returned from the Land Registry database. Zoopla also carries the same information for each postcode, although the data is not always up-to-date. Another option is to speak to the estate agent that handled the sale of the property. They will be able to tell you what the property sold for. You can also ask the current owner how much they paid for the property.
