Along with another monument at Westminster Abbey, Adam was commissioned to design a sarcophagus for the chapel at Alnwick, dedicated to memory of the Duchess of Northumberland who had died suddenly on 5 December 1776. Aymonino has found that Adam provided a new design for the lid of the sarcophagus in June 1781. The work was undertaken by Nicholas Read (c1787), who was paid £240, and continued until at least 1783.

This design is the only surviving graphic evidence for the monument, taking the form of a Roman sarcophagus, with a paired portrait of the Duke and Duchess in the middle. The executed object was very large, measuring 9 feet long, 4 feet 2 inches wide, and 3 feet 7 inches high, and was placed under Adam’s stained glass window in the chapel at Alnwick. Written descriptions of the sarcophagus describe something a little different to this design, and it is presumed that Read executed the design with some alterations to this drawing. The monument was lost when the chapel was demolished under Sir Anthony Salvin in the 1850s.