Cassiobury House was a country house, now demolished, which was in Cassiobury Park, in Watford, England.  Originally a Tudor building, dating from 1546 for Sir Richard Morrison, it was demolished in 1927.
At the Restoration, the 1st Earl extended the house; later, the 5th Earl added a Gothic exterior and an orangery, and redesigned the gardens. The 7th Earl married an American heiress, helping to maintain the estate. However, after the 7th Earl died, his widow sold up to pay death duties and the house was demolished for its materials in 1927. 
Cassiobury House was demolished in 1927. Carvings from the staircase are now in the Metropolitan Museum of Art. The stables remain and were converted into a retirement home. Part of the grounds, but not the site of the house, now form the public Cassiobury Park.
The dower house of the estate "Little Cassiobury" still exists. Having been designed by Lady Elizabeth Wilbraham (the first woman architect), and built in 1670 it is listed as "grade II*" and "at risk" by English Heritage. Under the supervision of the Portmeirion architect Clough Williams-Ellis there were renovations and extensions to the house in 1937-38. Soon after that Hertfordshire County Council bought Little Cassiobury in 1939 under a compulsory purchase order. They used part of its land to build Watford College, and they continued to use the house as an education office for most of the 20th century.
- England's lost Downtons Daily Mail
- "'Watford: Manors', A History of the County of Hertford: volume 2 (1908), pp. 451-464.". Retrieved 11 March 2012.
- Lost Heritage
- Staircase from Cassiobury Park, Herfordshire, The Metropolitan Museum of Art.
- Watford councillors delay Cassiobury Court decision Watford Observer, 2 February 2011
- Cassiobury Park
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