This monstrous family mansion has 250 rooms, an indoor pool a bowling alley and 3 kitchens. If you were advertising it for sale it would be a 35 x 43, which is really weird ... 35 bedrooms but 43 bathrooms! Anyway, more information here.
It could be argued that Windsor Castle is a private establishment and if you felt that the British Royal family owning such a house does not constitute public ownership, it would probably win the title of World's largest house. Windsor Castle has a floor area of about 45,000 square meters. However, I reckon this one is cheating ...
Anyway, if you happen to know of a larger houses anywhere in the world, drop me a comment (please, no temples, government buildings or office blocks ... just private houses).
Update: It would appear that the Biltmore house will sit at the top for only a short while longer. A new Biggest house in the world is rising in, of all places, Mumbai. Details about this monsterous house here.