When Hermione invited Draco, she had not anticipated the living arrangements. After Harry had moved in, he had put all his energy into making Grimmauld Place not only habitable but comfortable. He had moved to the top floor into Sirius’ old bedroom. It had seem fitting. He had offered Regulus’ old room to Ron who had refused to set foot in it. He had moved into Fred and George’s old room on the third floor instead. Andromeda had taken Mr and Mrs. old bedroom and they had converted Remus’ room into a nursery for Teddy. When Narcissa Malfoy came they moved her to Buckbeak’s room, although she had no idea of the previous occupant of her room. With Hermione’s arrival she had taken up residence in her old room. That left Malfoy with two choices. Either he had to move into Harry or Ron’s old room or move to Regulus’ room. Ron had been a downright prat, not wanting to Draco to stay in his previous room. So Malfoy had to climb all the way to the fourth floor, right next to Harry.