I want to know why God asked Hezekiah to set his house in order for he will die? Second, did Hezekiah actually set his house in order before he died after God added more years for him to live?
While most of us do not know when we might die, some of us do get advance warning which allows us to get our affairs in order so that our departure isn't a disaster to those we leave behind.
"In those days Hezekiah became mortally ill. And Isaiah the prophet the son of Amoz came to him and said to him, "Thus says the LORD, 'Set your house in order, for you shall die and not live.'"" (II Kings 20:1).
Apparently at this time Hezekiah had no heir. His son, Manasseh, was not born until three years after this time (II Kings 20:6; 21:1). There would be the need to select someone from among the descendants of David to take his place, transitions of government functions, as well as getting personal matters completed before his death. It was a sign of God's respect for Hezekiah that He gave him warning about his pending death.
Because of Hezekiah's prayer, God granted Hezekiah an additional fifteen years (II Kings 20:6), but as the story unfolds we find that Hezekiah's faithfulness is less. He accidentally sets in motion the downfall of Israel (II Kings 20:12-18). But instead of regret, Hezekiah's thought was that at least it would not happen to him. "Then Hezekiah said to Isaiah, "The word of the LORD which you have spoken is good." For he thought, "Is it not so, if there will be peace and truth in my days?"" (II Kings 20:19).
Though Hezekiah had fifteen years advanced warning, his twelve-year-old son succeeded him and he was among the worse kings in Israel's history. Hezekiah totally failed to teach his son to follow in his steps.
I suspect that things would have been a bit better for Israel if Hezekiah had not lived those extra years. But did Hezekiah set house in order? We don't know what he did, but we do know that he did not do enough.