The 130-year-old mansion is currently part of a state historic park near downtown Sacramento.
It was built by hardware dealer Albert Gallatin and was sold to the state in 1903 for $32,500. That's about $800,000 in today's money.
It was the home of 13 governors and their families.
Initially, the restoration project was supposed to return the third floor to what it looked like when the house's most recent residents, former Governors Pat Brown and Ronald Reagan, lived there.
But the discovery of a discarded chunk of plaster gave workers clues to what the floor originally looked like.
That will allow restorers to return the room to what it looked like before carpeting, room modifications and layers of paint and wallpaper obscured its original colors and design.