From the beginning, the renovation of the Bank Stock Office was constrained by the existing foundations of the hall. Soane placed his four piers precisely where four of Taylor's less substantial supports had been, and it appears that little was done to alter the existing cellar and foundations by Taylor.

The built design of the cellar (SM volume 74/12, SM volume 74/48 and SM volume 74/37) indicate a triple-aisled space, corresponding to the hall above, with a four-bay centre aisle flanked by seven-bay side aisles, these last corresponding to the seven bays of Taylor's original hall. Beneath the cellar floor, inverted segmental arches formed a foundation.

The series of preliminary designs (SM volume 74/5, SM volume 74/4 and SM volume 74/3), show Soane experimenting with converting the side aisles into four-bay spaces to conform to the centre aisle, but in the end Taylor's existing seven-bay side aisles were realised, possibly for reasons of economy. Soane probably did reinforce some of Taylor's buttressing. It would seem the cellar was used for storage; doors are indicated at the north and south ends. In the executed design, Soane placed in the centre a firebox to warm the stove in the hall above (see SM volume 74/34 and SM volume 74/35). The transverse sections also show the hollow-cone pot vaulting of the hall roof.