Its replacement was built in 1804 by his son , William Strutt , on the foundations of the old mill .
Is one of the oldest surviving examples of an iron - framed ' fire - proof ' building in the world .
Its replacement was built in 1804 by his son , William Strutt , on the foundations of the old mill and is one of the oldest surviving examples of an iron - framed ' fire - proof ' building in the world .