How do we calculate plusvalia on a house sale

If plusvalía is to be paid on a property, the cost is based on the land value when the property was bought against the land value of the property when sold. The town hall should be able to give you a rough estimate based on the "valor catastral" but they can't give you the actual bill until the property sale has been signed at the Notary. The vendor should pay the plus valía as it is he/she who is making any profit on the sale of the property.