To provide an ancillary structure which is collaterally connected to a building unit constituting a unit building for enlarging the interior space of the building unit, with flexibility for easy tracking to the building unit.

A small-sized ancillary unit 10F which is connected to an elevation surface of a rectangular parallelepiped building unit 1 constituting a unit building for enlarging the interior space of the building unit forms a rectangular parallelepiped comprising four column materials 11, four lower beams 12 and 13 for horizontally connecting the lower ends of the column materials, and four upper beams 14 and 15 for horizontally connecting the upper ends of the column materials. The four lower beams and the four upper beams are connected to the four column materials with fasteners. The connection with the fasteners may be performed by: bringing a joint member fixed to the end part of the lower beam or the upper beam in contact with the column material and detachably connecting them with the fastener so as to form a flexible structure; or bringing the joint member fixed to the end part of the lower beam or the upper beam in contact with the joint member fixed to the column material and detachably connecting them with the fastener.