Drawings 3 to 6 show the new south-east extension, having an entrance porch and library on the ground floor and a bedroom, drawing room and water closet above (Journal 3, p.181). The addition adjoins the 'old library' of the existing house. Drawings 3 to 5 were presented to William Pitt on June 4th 1796 and a design was settled on June 16th.

Drawings 3 to 6 are all dated 4 June, the date of Soane's meeting with William Pitt. It appears that the flier to drawing 3 was added after the drawings were presented and in response to Soane’s consultation with Pitt. Marks in pencil on the plans indicate the communications between the rooms and were probably made as the architect and client discussed the drawings. Drawing 6 is a copy of drawing 3, showing the same design as drawing 3 (except for the flier) and having the same erasure marks on the squared alcoves of the ground-floor bay window.

Drawings 4 and 5 show the interior of the library. The canted bay window has bookcases between the three windows. Facing the windows is a chimney-piece between four round-headed alcoves. The alcoves contain bookcases and a door, with the rounded recesses over each kept as a display for globes and busts. The library has green walls and red window treatments. Drawing 5 includes a tall gilded mirror over the chimney-piece.

Drawings 3 to 5 are in a folder with an estimate (SM 2/9/9a) for the building works as of June 1796: 'The new library, entrance & rooms over, / pulling down part of the old entrance &c / and making good the roof and clearing / away for the new room' is estimated at £950; 'To paint and white wash the whole of the house / (exclusive of offices), to repair the joiners work / and to put the whole into proper repair' is estimated at £200; and 'the portico before the old library' is estimated at £120, totalling £1270. The same total estimate is written by Soane on the verso of drawing 4, with a note that Mr Pitt has given his approval for the plans and estimate.