An add-on part on a vehicle body, where a plurality of body hinge halves are each installed on the vehicle body using a first reference point system comprising body reference points, at least one vehicle transverse axis coordinate of the installed body hinge halves is measured, a plurality of add-on part hinge halves are each installed on the add-on part by using the first reference point system and a second reference point system comprising add-on part reference points, at least one correction value for the add-on part hinge halves with regard to the transformed arrangement of the add-on part hinge halves in the vehicle transverse axis is determined from the measured vehicle transverse axis coordinate of the body hinge halves, and the at least one correction value is used in the installation of the add-on part hinge halves.