For separate structures such as a freestanding garden room, shed or carport, you do not need a permit, as long as the surface area remains below 40 m². Additional conditions stipulate that the structure should be no more than 3.5 m tall, built within a radius of 30 m from your home, 3 m away from your plot boundary in a side garden and 1 m away from your plot boundary in a rear garden.
Often, regional permit conditions also apply, so make sure you refer to the specific regulations for your freestanding pergola in Flanders, Wallonia or Brussels.