George Capell, 7th Earl of Essex married an American heiress, helping to maintain the estate.
The parties and entertainments at Cassiobury House continued into the new century: in 1902 it was visited by the young Winston Churchill and King Edward VII.
But at about this time the Essex family planned to let the house and live in London.
The upkeep was becoming increasingly expensive.
In 1909, 184 acres (0.74 km²) of parkland were sold, most to Watford Borough Council for housing and the public park.
After the 7th Earl's death (1916), his widow and her son, the 8th Earl, sold up in 1922 to pay death duties, and the house was demolished for its materials in 1927.

More land for the public park was purchased by Watford Borough Council in 1930.

Construction of the residential Cassiobury Estate began.
The land was made subject to restrictive covenants stipulating that only good quality detached or semi-detached houses would be allowed.
Most activity was in the 1930s, though building still continues, mostly of "infill" housing on former back gardens.
The park is bounded by Parkside Drive and Coningsby Drive on the north, and Cassiobury Park Avenue on the south.

On Thursday 8 June 1922, at 2.30 p.m. at 20 Hanover Square, "By direction of the Right Honourable Adèle, Countess Dowager of Essex", "Cassiobury Park estate including the historical family mansion, Little Cassiobury, and the West Herts Golf Links, embracing in all an Area of about 870 acres (3.5 km²)" was auctioned by Humbert & Flint, in conjunction with Knight, Frank & Rutley.

Having remained unoccupied and unsold, the house itself was demolished in 1927.
Only the stable block remains: this has been converted to Cassiobury Court, an old people's home, still extant in Richmond Drive.
The grand staircase (said to be designed by Gibbons but since attributed to Edmund Pearce) was removed to the Metropolitan Museum of Art in New York.
Other materials from the house were used to restore Monmouth House in Watford High Street.
Posters advertised "To lovers of the antique, architects, builders, etc.,
300 tons of old oak: 100 very fine old oak beams and 10,000 Tudor period bricks".

"English country house in New York" in Country Life on Tuesday, 21 July 2009, by Arabella Youens says that salvaged material from the old mansion was used to build a new Cassiobury House in the US:-
"When Cassiobury House in Hertfordshire, home to the Earls of Essex for more than 250 years, was dismantled in 1927 much of the masonry was used in the construction of a new house of the same name in Bedford, New York."

In 1967, even the quaint, castellated entrance gates on the Rickmansworth Road were demolished to make way for a new traffic system.

Little Cassiobury (Grade II* listed), the Cassiobury Estate dower house, still exists in Hempstead Road, Watford.
It was sold separately from the rest of the estate.
While it was in private ownership it was extended and renovated, in 1937, by the Portmeirion architect Clough Williams-Ellis.
Soon after that it was compulsorily purchased by Hertfordshire County Council, who built Watford College (latterly West Herts College) on part of the site.
Herts County Council used Little Cassiobury as an education office for most of the 20th century.
George Capell, 7th Earl of Essex married an American heiress, helping to maintain the estate. The parties and entertainments at Cassiobury House continued into the new century: in 1902 it was visited by the young Winston Churchill and King Edward VII. But at about this time the Essex family planned to let the house and live in London. The upkeep was becoming increasingly expensive. In 1909, 184 acres (0.74 km²) of parkland were sold, most to Watford Borough Council for housing and the public park. After the 7th Earl's death (1916), his widow and her son, the 8th Earl, sold up in 1922 to pay death duties, and the house was demolished for its materials in 1927. 
More land for the public park was purchased by Watford Borough Council in 1930.
Construction of the residential Cassiobury Estate began. The land was made subject to restrictive covenants stipulating that only good quality detached or semi-detached houses would be allowed. Most activity was in the 1930s, though building still continues, mostly of "infill" housing on former back gardens. The park is bounded by Parkside Drive and Coningsby Drive on the north, and Cassiobury Park Avenue on the south.
On Thursday 8 June 1922, at 2.30 p.m. at 20 Hanover Square, "By direction of the Right Honourable Adèle, Countess Dowager of Essex", "Cassiobury Park estate including the historical family mansion, Little Cassiobury, and the West Herts Golf Links, embracing in all an Area of about 870 acres (3.5 km²)" was auctioned by Humbert & Flint, in conjunction with Knight, Frank & Rutley.
Having remained unoccupied and unsold, the house itself was demolished in 1927. Only the stable block remains: this has been converted to Cassiobury Court, an old people's home, still extant in Richmond Drive. The grand staircase (said to be designed by Gibbons but since attributed to Edmund Pearce) was removed to the Metropolitan Museum of Art in New York. Other materials from the house were used to restore Monmouth House in Watford High Street. Posters advertised "To lovers of the antique, architects, builders, etc., 300 tons of old oak: 100 very fine old oak beams and 10,000 Tudor period bricks".
"English country house in New York" in Country Life on Tuesday, 21 July 2009, by Arabella Youens says that salvaged material from the old mansion was used to build a new Cassiobury House in the US:-
"When Cassiobury House in Hertfordshire, home to the Earls of Essex for more than 250 years, was dismantled in 1927 much of the masonry was used in the construction of a new house of the same name in Bedford, New York."
In 1967, even the quaint, castellated entrance gates on the Rickmansworth Road were demolished to make way for a new traffic system.
Little Cassiobury (Grade II* listed), the Cassiobury Estate dower house, still exists in Hempstead Road, Watford. It was sold separately from the rest of the estate. While it was in private ownership it was extended and renovated, in 1937, by the Portmeirion architect Clough Williams-Ellis. Soon after that it was compulsorily purchased by Hertfordshire County Council, who built Watford College (latterly West Herts College) on part of the site. Herts County Council used Little Cassiobury as an education office for most of the 20th century.
Cassiobury Park