Method for installing an add-on part on a vehicle body, in which - a plurality of body hinge halves are installed on the vehicle body, in each case by using a first reference point system comprising body reference points, - wherein the body hinge halves are each defined with regard to their arrangement in the vehicle longitudinal axis and in the vehicle vertical axis by the reference point system and - wherein a pivot axis of the add-on part is defined by hinge holes or hinge bolts of the body hinge halves, - at least one vehicle transverse axis coordinate of the installed body hinge halves is measured, characterized in that - a plurality of add-on part hinge halves is installed on the add-on part, in each case by using the first reference point system and a second reference point system comprising add-on part reference points, - wherein the add-on part hinge halves are each defined with regard to their transformed arrangement in the vehicle longitudinal axis and in the vehicle vertical axis by the second reference point system, - wherein the transformed arrangement comprises a reference to a finally installed, closed state of the add-on part, - wherein at least one correction value for the add-on part hinge halves with regard to their transformed arrangement in the vehicle transverse axis is determined from the measured vehicle transverse axis coordinate of the body hinge halves and - wherein the add-on part hinge halves are installed by using the determined correction value, - the add-on part is moved by means of a positioning device to a measuring position at a predefinable distance from the vehicle body, - at the measuring position, position and orientation of hinge bolts or hinge holes of the add-on part hinge halves are determined and are compared with position and orientation of corresponding hinge holes or hinge bolts of the body hinge halves, - by using a comparison result, the add-on part is suspended by means of the positioning device with its add-on part hinge halves on the body hinge halves installed on the vehicle body. - a plurality of body hinge halves are installed on the vehicle body, in each case by using a first reference point system comprising body reference points, - wherein the body hinge halves are each defined with regard to their arrangement in the vehicle longitudinal axis and in the vehicle vertical axis by the reference point system and - wherein a pivot axis of the add-on part is defined by hinge holes or hinge bolts of the body hinge halves, - wherein the body hinge halves are each defined with regard to their arrangement in the vehicle longitudinal axis and in the vehicle vertical axis by the reference point system and - wherein a pivot axis of the add-on part is defined by hinge holes or hinge bolts of the body hinge halves, - at least one vehicle transverse axis coordinate of the installed body hinge halves is measured, characterized in that - a plurality of add-on part hinge halves is installed on the add-on part, in each case by using the first reference point system and a second reference point system comprising add-on part reference points, - wherein the add-on part hinge halves are each defined with regard to their transformed arrangement in the vehicle longitudinal axis and in the vehicle vertical axis by the second reference point system, - wherein the transformed arrangement comprises a reference to a finally installed, closed state of the add-on part, - wherein at least one correction value for the add-on part hinge halves with regard to their transformed arrangement in the vehicle transverse axis is determined from the measured vehicle transverse axis coordinate of the body hinge halves and - wherein the add-on part hinge halves are installed by using the determined correction value, - wherein the add-on part hinge halves are each defined with regard to their transformed arrangement in the vehicle longitudinal axis and in the vehicle vertical axis by the second reference point system, - wherein the transformed arrangement comprises a reference to a finally installed, closed state of the add-on part, - wherein at least one correction value for the add-on part hinge halves with regard to their transformed arrangement in the vehicle transverse axis is determined from the measured vehicle transverse axis coordinate of the body hinge halves and - wherein the add-on part hinge halves are installed by using the determined correction value, - the add-on part is moved by means of a positioning device to a measuring position at a predefinable distance from the vehicle body, - at the measuring position, position and orientation of hinge bolts or hinge holes of the add-on part hinge halves are determined and are compared with position and orientation of corresponding hinge holes or hinge bolts of the body hinge halves, - by using a comparison result, the add-on part is suspended by means of the positioning device with its add-on part hinge halves on the body hinge halves installed on the vehicle body.