A mansion was a large house.
The Third Doctor and Sarah Jane met St. Nicholas in the garden of a New York City mansion. (PROSE: A Visit from Saint Nicholas)
Chase Mansion was destroyed when it was bombed in an air strike which killed the Krynoid that Harrison Chase had allowed to grow there. (TV: The Seeds of Doom)
The arrival of the Eighth Doctor and Charlotte Pollard at the mansion known as Edward Grove created a paradox, resulting in a time loop which allowed it to become sentient. (AUDIO: The Chimes of Midnight)
Ace burned down the mansion of Gabriel Chase in 1983 in anger over the attack on Manisha's flat. (TV: Ghost Light)
Caliburn House was a mansion known for being haunted for centuries by the Witch of the Well. (TV: Hide)