Watkin's main research interest has been classical architecture, particularly from the 18th century to the present day, and has published widely on that topic. He has also published on general topics including A History of Western Architecture (4th ed. 2005) and English Architecture: A Concise History (2nd ed. 2001), as well as more specialised monographs on architects Thomas Hope, Sir John Soane, James Stuart, and C. R. Cockerell.

Watkin first came to wide international attention, however, with his book Morality and Architecture: The Development of a Theme in Architectural History and Theory from the Gothic Revival to the Modern Movement (1977), re-published in expanded form as Morality and Architecture Revisited (2001). The basic premise of his argument is that the language with which modernist architecture is described and defended is rooted in the false notion of the Zeitgeist or “the spirit of the age”, as put forward by German Idealist philosopher Friedrich Hegel, so that any opposition to modernist architecture — and here he has in mind the revival of classical and traditional architecture, which he has championed in his writings - are condemned as “old-fashioned”, irrelevant, anti-social, and even immoral.

In terms of Zeitgeist architecture, he traces its moralistic attitude back to architects Pugin, Viollet-le-Duc and Le Corbusier, among others — including their supporters within history such as Nikolaus Pevsner, who claimed that their chosen style had to be truthful and rational, reflecting society's needs. Watkin also sees the pedigree of a distorting modernist architectural history emerging from Hegel, and that modern art and architectural history began in the nineteenth century as a by-product of history and the philosophy of culture in Germany and the rapid growth of Marxist sociology.

Among the ‘contemporary’ architects Watkin has championed are John Simpson and Quinlan Terry, as well as theorist Leon Krier. In his book on Terry, Radical Classicism: The Architecture of Quinlan Terry (2006) Watkin is forthright: “The modernism with which Quinlan Terry has had to battle is, like the Taliban, a puritanical religion.”

"it is man, creative, mysterious, and unpredictable, who is the proper subject of the historian, not the subterranean collective urges of the spirit of the age or of the 'needs' of an as yet non-existent society." (David Watkin, Morality and Architecture)