The vast rooms of Palazzo Raggi, an 18th-century architectural jewel a stone’s throw away from Rome’s Spanish Steps, were once lived in by an illustrious noble family and a host of cardinals and other historic characters hailing from Italian aristocracy. Now the six-storey building, which stood derelict for years, is being revived as it prepares to welcome a new class of inhabitants: the foreign billionaires who are turning their backs on the likes of Switzerland, Dubai or the Cayman Islands and moving to Italy to taste the dolce vita and, moreover, the financial benefits of a tantalising tax regime. Italy...