The octagonal room – probably containing Keate’s museum – was located at the rear of the house, within a new extension of 1777. See scheme notes.

In his poem of 1787, The Distressed Poet: a Serio-Comic Poem, Keate described taking Adam to court over a ceiling which had collapsed. His suit was not successful, but it can serve to tell us that at least one of Adam’s ceiling designs for 9 Charlotte Street was executed. The ceiling in question is described as Etruscan, and while none of Adam’s extant ceiling designs for the house make use of the characteristic black and terracotta colour scheme of an Etruscan design, there were Etruscan-style furnishings designed for the Octagonal room, so the ceiling in question was most likely in that room. Adam volume 12/117 is inscribed as unexecuted, but 12/119 is not, and while there is no way to prove that this design was executed, the surviving evidence suggests that it was.