A modular building system or storage system is disclosed with a number of stacks of containers as shown in FIG. 2, the stacks being positioned within a frame structure having uprights and a horizontal grid disposed above the stacks, the grid having substantially perpendicular rails on which load handling devices can run. Containers having functions associated with a number of residential or commercial uses are moved in to and out of the stacks by the robotic handling devices running on the grid. The containers disposed in the stacks are selected by function on demand by a user. In this way the building is reconfigurable to take in to account required uses.