A revaluation “should” occur whenever a property is extended, reformed etc. However, the revaluation is dependant on the information being given to the Catastro by the equivalent dept in the Ayuntamiento local to the property and the info being given to the Ayuntamiento in the first place by the owner!
Once an escritura de obra nueva has been completed a copy should go to the catastro department of the Ayuntamiento who then passes the details to the provincial catastro office.
Eventually a new valor catastral will be issued. The catastral record will also show the increased m2 if appropriate.
Of course most people don’t bother, as a higher valor catastral = higher taxes, IBI etc.
However, I have heard that there are periodic revaluations of properties being carried out by Catastro in some areas, resulting in requests for back taxes etc.