Albert Barnes Commentary 1 Kings 6:16

"And he built twenty cubits on the hinder part of the house with boards of cedar from the floor unto the walls [of the ceiling]: he built [them] for it within, for an oracle, even for the most holy place." — 1 Kings 6:16 (ASV)

The meaning is that at a distance of 20 cubits from the end wall, measured along the side walls of the house, Solomon constructed a partition that reached from the floor to the ceiling and had a doorway in it. In this way, he created a sanctuary within the house to serve as the Holy of Holies.