While in Canterbury, Sandys became a keen antiquarian, especially relating to those antiquities of the historic county of Kent. In 1844, he became an early member of the British Archaeological Association, out of an interest in Kentish archeology. In 1845, he presented an impression of a brass seal, from the time of Edward III, on 23 December. The following year, he published his first book, A Critical Dissertation on Professor Willis's "Architectural History of Canterbury Cathedral" (1846). The work was dedicated to upholding William Gostling's view of the structural history of Canterbury Cathedral, against Robert Willis' "fanciful hypothesis", that the cathedral was almost entirely built in Archbishop Lanfranc's time, after the Norman conquest. J. G. Waller (in the Journal of the British Archeological Association) reviewed it favourably, claiming it "is exceeding useful and will be read with profit by all who feel interested in the metropolitan church of Canterbury", praising it for correctly pointing out flaws in Willis' work. Modern biographer of Willis, Alexandrina Buchanan, has taken a less positive view of the work, declaring that "in reality, it was Sandys who was guilty as charged [in making incorrect historical deductions], though Willis did not condescend to make this point".
