Prior to it's obliteration, Jane's mansion was a behemoth. Towers upon towers, and storeys upon storeys. It goes (at the lowest point) 100 feet down into the ground. It contains an underground laboratory, which is suspended over a 10 storey cavern. Jane's backyard is like her sisters, there are cliffs over the ocean. She has hundreds of underground tunnels dug into the side of the cliff, which span the entire city, allowing her to maneuver the whole city more easily. Currently (as of the season 4 finale), Jane, Antoinette, and Mandy are living in it, with Mozart locked down in the lab. The dead bodies of Marina, David, Jamison, and Charles, are all at the bottom of her lab's pit. The house is AT LEAST twice the size of her sister's.
This house is actually inherited from Jeremiah's mother and father. It is their old mansion, passed down to Jane. In the season five finale, Mozart did to Jane's estate, exactly what he did to Antoinette's... How blew it apart with dynamite.