This paper is based on the authors’ longstanding interest in Northumberland House and follows two previous papers by Manolo Guerci, which appeared in this journal in 2010 and 2014 respectively. The first paper explored the house as originally built by the 1st Earl of Northampton between 1605 and 1614, while the second looked at both the ownership of the Earls of Suffolk, in the years between Northampton’s death and 1642, and the transformations of the 10th Earl of Northumberland, from that year to 1668. The changes initiated by the 7th Duke and Duchess of Somerset in 1748, and completed by the 1st Duke and Duchess of Northumberland (third creation) in the 1750s and 1760s, finished the process of shifting the public side of the house from the Strand to the river side, begun by the 10th Earl of Northumberland. Never before fully investigated, this period is crucial in the long history of the house, as a large body of renowned craftsmen and builders (presented as supplementary material to the online edition of this paper) experimented with lavish interiors, which became a model for contemporary enterprises. In addition, Northumberland House acted as the showcase of the couple’s taste and patronage, as well as the venue for the private ‘Musaeum’ of the Duchess, within a larger remarkable collection, and probably functioned as a proto-academy for selected artists and connoisseurs.