Spain will launch a new house price index, to be published quarterly by the National Institute of Statistics (INE) starting 1 October.

The new house price index will be based on property transaction prices witnessed by notaries and included on the deeds inscribed in Spain’s property register.

Based on actual transaction prices, the new house price index is expected to be more useful than the quarterly house prices figures published by the Ministry of Housing, which are based on mortgage valuations. “The prices used will refer, in real time, to actual transaction prices, not theoretical valuations,” said a statement from the INE.

That said, the new index will only be as good as the figures provided by the notaries, which are never entirely accurate due to the common practice of paying for property with cash under the table. This practise of under-declaring property prices on the deeds of sale (to reduce taxes) is declining, but it is still prevalent enough to distort the figures.

Unlike the Ministry of Housing’s figures, which also give average property values per region in Euros per square metre, the new index will only show the percentage change in property prices over time. It will, however, also track changes in new build and resale property values.

The new index has been developed by INE as part of a European initiative lead by Eurostat – the EU’s statistics office – to facilitate transparency and comparisons between EU housing markets.