Successive Prime Ministers have 'punted the ball' on this due to attitudes such as yours. 

Do you think the U.S. simply avoids spending money on the upkeep of The White House? And aren't the British spending money on renovating Buckingham Palace?

The current P.M. doesn't "own" 24 Sussex since it is public property. And there has been numerous articles over the years about the building falling well below current standards in many respects. So in what sort of poor state of repair do you think 24 Sussex needs to be in before major repairs are undertaken?