I’ve seen a derelict house I am interested in how do I find

There’s a derelict house I have seen and has been empty for a few years now how do I find out who owns it.

Spanish land registry:
https://www.sedecatastro.gob.es/

A nota simple will tell you what you need to know.

Pete.

What is that?

You need to find the reference for the house, you can do online on the link Pete told you. Then you can buy online a "nota simple" or go the "Registro de la Propiedad" where that house below. In the "nota simple" the goverment tells you the name of owner of the house.

Then is your work to look for that owner.

Thank you sorry did not read Pete’s answer fully.