The "valor catastral" does not normally change, I think. I´ve been getting the same value for my house for the last 6 years.
it is fix by the Council, and it is the value of the land plus the construction but also, they take into consideration the services in the area (i..e. schools, hospitals, etc,). Normally, we say in Spain, it is approximately half the value in the market for that house (which I am not sure if this is really true)
The Valor Catastral for my 3 bedroom apartment is:
v. suelo (land value): 18.317€ v.construc. (value of the construction): 26.561€ euros equals 44.878
The annual IBI fee for 2010 was 103,79 €