The invention relates to a construction apparatus having an extendable mast with an upper mast element and a lower mast element, whereby the upper mast element is longitudinally displaceable relative to the lower mast element. The apparatus according to the invention is designed with a linear drive for displacement of the two mast elements relative to each other, whereby the linear drive has an upper drive part which can be actuated in a linear manner relative to a lower drive part of the linear drive, and with a locking device for locking the two mast elements in an extended mast position. In accordance with the invention provision is made for the upper drive part of the linear drive to be fixed to the upper mast element, for the lower drive part of the linear drive to be displaceable longitudinally of the lower mast element and for a securing device to be provided on the lower mast element, with which the lower drive part of the linear drive can be secured in a releasable manner to the lower mast element for displacement of the upper mast element. The invention further relates to a method for operating a construction apparatus with an extendable mast, which can be carried out in particular with a construction apparatus in accordance with the invention.