The Naka-zashiki (Spare Room) Building is sandwiched between the Omote-zashiki (Parlor) Building and the Oku-zashiki (Inner Parlor) Building to the north and south, and the kitchen wing is to the east. The only entrance to this building is on the west side so there is little exposure to light. As there are no nageshi (decorative horizontal timbers) like in either the Guest Room and Akitake's room, it is not as high in status. We have learned from an examination of the shape of the roof that this building is an extension. For whatever purpose, it seems to have been wedged into the small space here.

Akitake Tokugawa’s six children were born in Tojo-tei House. Not all of them lived in Tojo-tei House full time, but an increase in the number of children would have created a need for additional rooms. It was probably used by Akitake's children, then as a clothing room and dressing room.