Albert Barnes Commentary 2 Chronicles 3:9

"And the weight of the nails was fifty shekels of gold. And he overlaid the upper chambers with gold." — 2 Chronicles 3:9 (ASV)

The upper chambers - (Compare 1 Chronicles 28:11). Their position is uncertain. One theory places them above the Holy of Holies, which was ten cubits (fifteen feet) lower than the main building (compare 1 Kings 6:2, 20).

Another view, accepting the porch's height of 120 cubits (2 Chronicles 3:4), regards the “upper chambers” (or “chamber,” Greek: ὑπερῷον, huperōon, from the Septuagint) as a lofty structure built over the temple entrance. A third suggestion is that the chambers were simply the uppermost of the three tiers of rooms that surrounded the temple on three sides (see 1 Kings 6:5–10).

This last explanation seems to be the simplest and best, though it does not explain the reason for the rich ornamentation of these rooms or for David’s special instructions concerning them.