The design is for a nine by five bay building with a front entrance into a hall and thence to the lecture room / library and large lecture room / music room of the ground floor. A side entrance takes the visitor upstairs to the museum and picture gallery via a geometric stair on a D-plan. SM 71/3/14 and SM 71/3/15 are alternative designs for the Great Lecture Room and Music Room. Both are of the same dimensions but with the seating arranged differently. SM 71/3/15 has windows on three sides and is shown as a lecture room. SM 71/3/14 has windows on two sides and is shown as a theatre with an orchestra pit. The interior architecture differs in that, for example, SM 71/3/14 (theatre) has a vaulted ceiling and SM 71/3/15 (lecture theatre) a canopy (or handkerchief) ceiling.

The room labelled 'Museum' (SM 71/3/13) has a plan and form reminiscent of an unexecuted design by Soane (and/or George Dance) for the picture gallery (sic) at Fonthill Splendens, 1787 (q.v.). The long narrow plan that was unavoidable at Fonthill was here more apt for museum objects. George Dance's executed design for the Common Council Chamber of the Guildhall, London of 1777-8, included a top-lit pendentive or canopy dome (in which the dome and the pendentives are part of the same sphere) similar to the Fonthill example though circular rather than elliptical on plan. The three lanterns of the Senate scheme are elliptical on plan but with flat roofs rather than the conical lanterns set on drums of the Fonthill example. The first built example of a related design was the fine canopy dome with lantern in the Yellow Drawing Room at Wimpole Hall, Cambridgeshire, 1790-1 (q.v.).

For the picture gallery design (SM 71/3/12), the young draughtsman has enthusiastically filled the frames with a great many joyous figures with flying draperies (or none at all) depicting various scenes from the Classics including the Rape of Europa.