The worker found the Iron Age soutterrain as he dug the foundations for a new property in Ness.
Archaeologist Chris Barrowman, who lives on Lewis, is surveying the chamber with his wife Rachel.
Mr Barrowman said food may have once been stored in the stone-lined structure.
The archaeologist told BBC: “They are usually associated with what are known as Atlantic roundhouses, or wheelhouses, of the later Iron Age.
“If this one was associated with a roundhouse it is likely to have been cleared away by now.”
Mr Barrowman said the well-preserved structure is the sixth to be recorded in the area.
The soutterrain would most likely be filled in and covered over to preserve the archaeology following a full survey, the archaeologist said.
Construction of the new house would then continue as normal.
However, the local authority archaeologist is due to liaise with the islander building the house on the way forward.