After the death of Princess Amelia the Gunnersbury estate passed through a number of owners till John Morley, a manufacturer of floor coverings, decided in 1801 to demolish the old Gunnersbury House and sell the land off as thirteen separate plots for development. Twelve of these were bought by Alexander Copland, who then built the Large Mansion. The remaining plot, in the North-East corner of the estate, was bought by Stephen Cosser who sold it to Major Alexander Morrison in 1807. Gunnersbury House, now known as the Small Mansion, was built there around 1806-9.

Like the Large Mansion, it is a long, rectangular building in the neo-classical style, but with oriental influences (such as the row of cast iron bells decorating the South front).

In 1828 the Small Mansion was sold to Thomas Farmer and remained in the Farmer Family until 1889. In this year it was bought by Leopold de Rothschild, who knocked down the dividing wall and re-united the two estates. The Rothschilds then used the Small Mansion as spare rooms for their many illustrious guests, including Edward Vll. They retained the Farmer family servants, including James Hudson, one of the most outstanding gardeners of his day and encouraged a friendly rivalry between Hudson and Mr Reynolds, who looked after the Large Mansion estate.

The long Terrace Room, which overlooks the South lawns, is available for hire for parties and wedding receptions.