The invention relates to an elevator, comprising a hoistway with a plurality of hoistway sections attached to each other to provide a hoistway with a hoistway space, an elevator car arranged in the hoistway space to be vertically movable in the hoistway space between door openings at landings of a building, and doors at the door openings. In order to avoid problems caused by a mutual change of height of a hoistway and a building, a sensor provides to a control system a signal indicating a level difference between a first reference point of the hoistway and a second reference point of the building.