Abstract: A transportable building module includes a first structure for defining a first enclosed space when erected at a building site and at least one elongate structural component having at least two elements that are relatively linearly slideable for linearly extending the structural component. The structural component is part of or attached to the first structure and is upright when the first structure is installed at the building site, whereupon the structural component is linearly extensible to provide part of a second structure for defining a second enclosed space above the first space.