The Golden Gardenia was a hotel located at 4653 California Lane, San Francisco, California in the Haight-Ashbury District in the 19th Century.
After the person who inherited it sells it to pay the inheritance tax, Rose Green buys it with her entire life savings in 2000 in order to accomplish her dream of opening a Bed & Breakfast, which was the trend in the area at the time. All that she and her friend Abby knew about the house was that it originally belonged to an "E. Valdez".
Throughout Nancy's investigation into the history of the house, she finds out that the E. Valdez was actually Lizzie Applegate, married to Diego Valdez (who was otherwise known as the robber El Diablo). Elizabeth probably had the house built around 1880 and owned it for around 25 years, waiting for Diego to return from his missions, but he never did, as the money used to buy the house in 1906 went to the Ladies' Protection and Relief Society, as mentioned in Lizzie's will was to happen if Diego was not found.
After Nancy's investigation concludes with the discovery of the Christmas Gold hidden in the house, Rose and Abby name the hotel The Golden Gardenia again and business takes off.
The cover of Message in a Haunted Mansion seems to depict the back of the house in its original state, without the porch on the side and with an extra tower. The cliff also indicates that it is a "house high above the sea", as The Bandit's Treasure says.