The following is stated in 'Regulations concerning the use of vehicles':

Loads overhanging the front of the car or projecting more than 1 m from the rear of the car, must be clearly marked at the outer ends with something that is alternating red and white in colour and has a visible surface area of at least 250 cm². The red material should be light-reflecting.

When the transport is unplanned, you can use other ways of marking the load.

If it is dark or if visibility is otherwise restricted, loads projecting more than 1 m from the rear of the car must also be marked with red lights visible from behind and from the sides. The lights should be clearly visible from a distance of at least 150 m, but not so bright as to be blinding for other road users.