Within built-up areas, parked motor vehicles and trailers weighing over 3.5 tonnes must be marked with a 
warning sign | .
The reflective, square parking warning sign is attached to the side of the vehicle facing the carriageway with the red triangle pointing upwards. The maximum permitted height must be observed when attaching the sign.
§ 51c StVZO
(1) Parking lights and parking warning signs indicate the lateral boundary of a parked vehicle.
(2) The following may be fitted to motor vehicles, trailers and trains:
- one forward-facing parking lamp for white light and one rear-facing parking lamp for red light for each side of the vehicle, or
- one marker lamp and one tail lamp, or
- 3. one removable parking lamp for white light for the front and one removable parking lamp for red light for the rear, or
- one parking warning sign for the front and one for the rear of the vehicle or train, each with 100 mm wide red and white stripes running outwards and downwards at 45 degrees.
On vehicles no wider than 2000 mm and no longer than 6000 mm, both the parking lamps referred to in point 1 on each side of the vehicle and those referred to in point 3 may be combined into a single unit.
(3) The lamps referred to in paragraph 2 sentence 1 numbers 1 and 3 and sentence 2 must be fitted to the vehicle in such a way that the lowest point of the illuminating surface is more than 350 mm and the highest point of the illuminating surface is not more than 1 500 mm from the carriageway. The outermost point of the illuminating surface of the lamps shall not be more than 400 mm from the outermost point of the outline of the vehicle.
(4) The lamps referred to in paragraph 2, sentence 1, number 3 shall be capable of being connected to the vehicle electrical system during operation or shall be equipped with rechargeable power sources which must be permanently connected to the vehicle electrical system when the vehicle is in motion.
(5) Parking warning signs, the effective parts of which may only be visible when vehicles are parked, must be mounted on the side of the vehicle or train facing the traffic as low as possible and not higher than 1,000 mm (highest point of the illuminated surface) so that they are flush with the outline of the vehicle, train or load. Deviations of no more than 100 mm inwards are permitted. Reflectors and licence plates must not be obscured by parking warning signs.
Quelle: https://www.gesetze-im-internet.de/stvzo_2012/__51c.html (translated)
