There have been several cases of aircraft hitting structures on the ground. Usually it happens when the aircraft is crashing, and there is a house there. For example, there have been instances of aircraft departing Santa Monica airport and losing power, winding up on a house that someone (perhaps stupidly) built near the departure end of the runway. But I think what you’re asking about is how often aircraft that are not going down crash into buildings.
Not very often. Sure, there have been suicides such as the guy who flew his airplane into the White House and the other guy who flew into his ex-girlfriend’s house; but aircraft vs. buildings are pretty rare. What happens more often is “controlled flight into the ground”. In this type of crash the pilot if often flying in IMC and is at an improper altitude. There are many accounts of pilots who got too low on their final approach to the runway and hit hills or trees in instrument conditions or darkness. (Hint: If the runway lights suddenly go dark, it’s probably not an electrical failure. Add power and pull up!) In these crashes the aircraft is under control and the pilot just flies it into an obstacle. The B-25 crash into the Empire State Building is an example of “controlled flight into an obstacle”, but more often the obstacle is the ground (hill/mountain/other terrain).
This seems to be the case in the recent Milan crash. The pilot reported a problem with his landing gear. A distracted pilot might not pay attention to Rule Number One: “Fly the airplane.” It’s also possible that the pilot in this case died before his aircraft crashed. He was 75 years old and could have had a heart attack. Too early to tell.