According to the Ben Ish Chai, Charvona provides the seemingly unnecessary location of the gallows the king presumably already sees in order to imply that Haman was usurping Achashverosh’s power by executing people in his own property.
R’ Chaim Kanievsky points out that, grammatically, the adjective describing the gallows should have preceded the preposition describing the gallows’ location. Charvona mentions the gallows’ height of 50 amos after mentioning Haman’s house. Furthermore, Charvona seems to say that the gallows are inside Haman’s house. In explanation, he quotes the Yalkut that suggests that the beam Haman used for the gallows was made from the crossbeam of his own house. In his hatred for Mordechai, he destroyed his own house. Throughout history, people have behaved in a self-destructive manner in attempt to fulfill their own temptations.