Surface treatment installation for the surface treatment of vehicle bodies with: a) at least two floor-bound transport carriages, each of which has a fastening device to which vehicle bodies can be fastened, wherein each transport carriage can be moved independently by means of its own entrainable drive; b) a treatment space which is delimited in a substantially gas-tight manner by a housing and in which the surfaces of the vehicle bodies can be treated and in which at least a portion of the fastening devices and the vehicle bodies fastened thereto can be moved at least temporarily in the treatment space by means of the respective transport carriage c) a moving area arranged outside the housing, in which the transport carriages are at least temporarily movable, wherein the respective transport carriage is located in the moving area when at least a portion of the respective fastening device and the vehicle body fastened thereto is located in the treatment space; and d) a gap formed in the housing between the moving area and the treatment space and through which the fastening device of the respective transport carriage projects when the transport carriage is located in the moving area, wherein a first atmosphere can be generated in the moving area and a second atmosphere different from the first atmosphere can be generated in the treatment space characterized in that the at least two transport carriages can be moved in a trackless manner at least outside the moving area, in that the surface treatment installation has a monitoring device arranged on the transport carriage which has a monitoring area U for monitoring the distance of the transport carriages from objects and/or persons, in that the size of the monitoring area and/or the position of the monitoring area U relative to the transport carriage is variable, and in that the monitoring area U is smaller when the transport carriage is in the moving area. a) at least two floor-bound transport carriages, each of which has a fastening device to which vehicle bodies can be fastened, wherein each transport carriage can be moved independently by means of its own entrainable drive; b) a treatment space which is delimited in a substantially gas-tight manner by a housing and in which the surfaces of the vehicle bodies can be treated and in which at least a portion of the fastening devices and the vehicle bodies fastened thereto can be moved at least temporarily in the treatment space by means of the respective transport carriage c) a moving area arranged outside the housing, in which the transport carriages are at least temporarily movable, wherein the respective transport carriage is located in the moving area when at least a portion of the respective fastening device and the vehicle body fastened thereto is located in the treatment space; and d) a gap formed in the housing between the moving area and the treatment space and through which the fastening device of the respective transport carriage projects when the transport carriage is located in the moving area, a first atmosphere can be generated in the moving area and a second atmosphere different from the first atmosphere can be generated in the treatment space characterized in that the at least two transport carriages can be moved in a trackless manner at least outside the moving area, in that the surface treatment installation has a monitoring device arranged on the transport carriage which has a monitoring area U for monitoring the distance of the transport carriages from objects and/or persons, in that the size of the monitoring area and/or the position of the monitoring area U relative to the transport carriage is variable, and in that the monitoring area U is smaller when the transport carriage is in the moving area.