[5] It is probable that Alexander's specialisation in the design of large industrial buildings and warehouses influenced Wallen's career.
While Wallen produced only a few prominent buildings, his practice not only undertook projects requiring huge budgets but also offered an exceptional standard of training for his pupils.
[7] In 1813, Ferry and Wallen produced a report for the repair of Norton Folgate workhouse and in 1818, they built Myddelton House, Bulls Cross, Enfield for Henry Carrington Bowles the last Governor of the New River Company.
William is credited with two similar architectural projects; the non-conformist chapels in Newbury, Berkshire and Newark, Nottinghamshire, built between 1822 and 1823.
At a time when the use of steel was treated with suspicion by many architectural theorists, his designs used "interior metal skeleton construction".
[18] Not only did this innovation allow Wallen to create warehouses with improved natural light and open space, it also gave him design flexibility in the emerging Neo-Classical era.
[21] In 1856, a witness to a burglary at 11 Spital Square told the Old Bailey "[John Wallen] is a great invalid, and not able to attend to his business".
However, his son, John James Wallen (1818-c.1865), an auctioneer, continued his father's interest in the development of London, publishing both criticism of the Bishopsgate Rail Terminus and advice for the improvement of railway shares.