The French property market is not really a speculative market with the exceptions of certain large cities, at least not in the same way as the UK where 90% of the population are home owners. Also the French homes again in general family homes and lived in sometimes for generations. INSEE the national statistics office keep very accurate figures of all transactions, as do of course the Notaires who report their figures to INSEE, they also have precise figures regarding square metrage and plot size. It is these figures that are used to estimate property values in France. For a particular village they can tell you exactly on the basis of recent sales the exactly per m2 value of a property on that street. It is then that the Estate Agent judges simply the condition and state of repair of the house which can vary the price by 5 to 10% at most. The plot size is also critical in judging the value too. Their exist also certain property or regional tendencies that need to be taken into account. The TGV arriving in Dax in 2017 for example would have a tendency to increase house prices in that region. Web site can give a very accurate idea of base value per m2. Efficity or Seloger or Notaires etc...