The rules are:
For local elections and european elections you can vote if you are empadronated.
For all other elections (regional/national) you are not eligible to vote.

It is, in my opinión, completely unfair. For example there are over 1 million registered foreigners in Catalunya who are legally here and pay their taxes. This accounts for 15% of the population. We have no right to vote in the regional or national elections.

From my understanding it is due to European rules, not Spanish.