Peter Ackroyd stressed the fact that Hawksmoor is not a historical novel in the strict sense of the word: "I have employed many sources in the preparation of Hawksmoor, but this version of history is my own invention." The novel "radically subverts the conventions of historical fiction" due to its idiosyncratic time structure.
Nevertheless, Ackroyd uses historical characters, sites and occurrences in his book. Nicholas Dyer, the architect of the seven churches, is modelled on Nicholas Hawksmoor but doesn't share his death date (Dyer disappears in 1715, Hawksmoor died in 1736). As said in the novel, the Commission for Building Fifty New Churches, which had been established by an Act of Parliament in 1711, commissioned Hawksmoor to build six churches, all of which are dealt with in the novel:
- Christ Church, Spitalfields,
- St George's, Bloomsbury,
- St Mary Woolnoth,
- St George in the East,
- St Anne's Limehouse,
- St Alfege Church, Greenwich.
The only fictional church in the book is Little St Hugh, venerating the historical Little Saint Hugh of Lincoln.
Hawksmoor was indeed assistant of Sir Christopher Wren, who is shown in the novel, historically correct, as profoundly interested in science and an active member of the Royal Society. Wren studied anatomy through dissection and vivisection so that the dissection of a killed woman by Wren in the novel refers to actual actions by Wren but maybe not on human beings. It is not certain but possible that Christopher Wren visited Stonehenge. The third historical character in the novel is Sir John Vanbrugh.
Historical occurrences Ackroyd refers to are the Great Plague of London of 1665/1666 and the Great Fire of London of 1666. Bedlam was actually not only a lunatic asylum but also an attraction for paying guests in the 18th century. For a penny visitors could look into the patients' cells, view the freaks of the "show of Bethlehem" and laugh at their antics. In 1814 alone, there were 96,000 such visits.