Abstract: A building structure comprises an outer shell of adjustable dimensions to define a substantially enclosed space. A section at least of the outer shell has a rigid lower frame including structural beams permanently mounted in foundations and defining a lower shell portion. A rigid upper frame defines an upper shell portion and includes structural beams arranged slidably to engage the structural beams of the lower frame. Each of the beams is formed with a longitudinal side flange and is arranged in slidably engagement pairs. Guide means are provided for fitting around the edges of adjacent side flanges of the beams for holding the beams in sliding engagement. Bolts or clamps are provided for fixing the upper portion relative to the lower portion in the raised position at least.