A construction apparatus having an extendable mast with an upper mast element longitudinally displaceable relative to a lower mast element. A linear drive displaces the mast elements relative to each other, and has an upper drive part linearly actuable relative to a lower drive part of the linear drive, and a locking device for locking the two mast elements in an extended mast position. The upper drive part is fixable to the upper mast element, the lower drive part of the linear drive is displaceable longitudinally of the lower mast element and a securing device is provided on the lower mast element, with which the lower drive part is releasably securable to the lower mast element for displacement of the upper mast element. A method for operating a construction apparatus with an extendable mast can be carried out with a construction apparatus in accordance with the invention.