We want to buy an old ruin... how do we find out who owns it???

Commune - planning department.

The commune hold the land registry record. Alternatively go and ring the doorbell of a neighboring property, or try the local notaire.

Every house or property is registered in Belgium.
The place to find the information is the local commune where they'll identify the property on a map, give you the cadastral number and the name (and address) of the person who is supposed to pay the property tax.

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