[2] One of his most famous works[1] is the unusual towered The Cloisters in neighbouring Letchworth Garden City, planned as a theosophical meditation centre and open-air school and which opened in 1907.

Cowlishaw married Edward Garnett's youngest sister, Lucy, in April 1897.

[4] At the end of World War I, like many Arts and Crafts architects of the period, he was commissioned by the Imperial (now Commonwealth) War Graves Commission to design memorials and cemetery layouts in Flanders and France under Sir Frederic G. Kenyon, the commission's advisor on architecture and layout.

[5] His work included the Pozières Memorial, in the Somme department (completed in 1930), to commemorate the missing of the Fifth and Fourth Armies, killed in 1918.

Among the smaller cemeteries he designed were Prowse Point, Rifle House and Devonshire, all around the area of Ypres.