In most western european countries, that's not a valid reason to raid someone's home. You have to have probable cause, the actions undertaken by the authorities must be prescribed by law, and they must be reasonable and proportional to the ends received. They must respect the country's internal laws and also the human rights laws of the European Union (which Italy has ratified and made part of their system of laws).
If these criterias aren't met, then the citizen can get compensation, and the public officer will most likely be relieved of his or her duties.
However, it seems Italy doesn't give a flying f*ck about any of this
My point, though, is that they didn't just raid Conte's house because he's a witness. He's definitely a suspect, and probably a major one since they went to such lengths.